Like Melissa said, growing a climber without support depends on the rose. For years, I grew Quadra without support because I was too lazy to work out a trellis or arch. In 4-5 years, it was a massive aggressive free standing bush with killer canes reaching in every direction. Quadra really wants to climb in cold zones, so it was clearly mad at me for not giving it territory to conquer. Once I set up an arch last year, Quadra shot up canes aggressively (there's that word again) all over the arch. In the meantime, the base of the bush was a stiff shrubby mess and I didn't even try to train most of the canes into good horizontal climbing position. I did notice that Quadra put out its best blooming season by far once I got the arch put up, indicating that it had been waiting for the chance to be horizontal and bloom off those lateral canes finally. This year, I was able to train some of those wayward inner canes toward the sides of the arch and it'll be much more tidy this year.

The opposite effect happened when I tried to grow Crown Princess Margarita without support, not realizing how much it preferred climbing. For its first three years, it literally lay on the ground with wimpy lax canes in every direction with blooms only on the end of the canes, and few of those. I kept waiting for it to put up laterals, since it was nothing but horizontal, but it never did in that configuration. In its fourth year, I wound it around a wide pedestal and it took off. It was a mass of blooms and clearly much happier around the pedestal than on the ground. True, it was its fourth year and climbers take that long to come into their own, but if it was going to put out some laterals it would have done that in the three years preceding. Every year, Vick's Caprice drops a cane or two to the ground beside its bush and little teeny laterals bloom like Christmas lights all along its length.

In the middle between those two are climbers like Teasing Georgia. It would prefer to climb on something, but seems to be happy in two spots in my yard having large arching canes that sort of fountain out from the base. Being an Austin, it's happy getting its arms whacked off periodically mid-season and coming back to bloom, so the unsupported state works OK. If I didn't do intermittent pruning, it would flop all over its neighbors and be too untidy even for me.

As for running a climbing rose through another rose, I think the competition would be too much in three ways. One is the root system, as Melissa mentioned, and both roses would want that coveted surface position for the best nutrients and water. Whichever one was strongest would probably win out, unless their bases were far enough apart. More than that in my world, though, is the sunlight. Roses want sun on the full length of their canes to bloom, particularly climbers in my experience, and if the climber was happy and getting all the good sun it would naturally shade out the base plant. You might get a few blooms from the underneath plant if it survived, but only one of the roses could be happy in that situation if they're going to spread out as they like. Clematis and roses can co-exist when happy because the clematis vine is so narrow it can work its way within the rose canes, and it likes to have shaded feet. The third way I think they'd compete is sheer weight. A happy climber needs a very sturdy stable support, and the supporting rose would risk getting its canes snapped by the weight of the top rose plus wind plus poor sun/nutrients to support the vigor of its canes anyway.

Having said all that, I'm sure it's possible and under the right conditions and combinations of roses could be done. My mini-climber Jeanne Lajoie wants to eat up vertical space but maintains thin enough canes that it might be able to not kill a rose under it. At present, it's duking it out with a very vigorous clematis, and the loser is the shepherd's cane I naively thought would hold up that combination!

Cynthia

Romainx87 - your other question - "could you maintain a climbing rose into a rose bush?" - I would not advise trying that. Most climbing roses want to climb. They take 3-4 years to get going, but then they put out very long climbing canes (which takes a lot of energy). Constantly cutting off those canes (and it would have to be constantly - they can be very aggressive) would not make them want to settle down and be a bush. It would just make them put out more long long canes. Some roses which are not really large climbers, but are rather just normally tall bushes, might be OK, but not what I think of as "real" climbers.

A general comment - in addition to paying attention to which rose it is, and learning about its normal growth habit, you should also pay attention to which roses like growing in your climate in the first place. Despite what you may see in books, not all roses like to grow everywhere. If you are careful to try roses which are happy in your type of climate (not just heat/cold, but humidity, soil, etc), that is 90% of the battle.

Curious - are you just curious, or are you trying to figure out what rose to use for a certain purpose? If the latter, and if you tell us approximately where you are geographically, you will get a lot of good advice on here about that.

Jackie

This is my morning to mention Austin's Mortimer Sackler, it seems! Well, it is a climber that I have been growing as a tall shrub for 5-6 years now. Stays strong, sturdy, and upright to about 8-10 feet, the height at which I trim an occasional branch now and then so that it won't become a climber. Pink bloomer and disease-resistant.

Kate

As stated, it's going depend a lot on which rose it is. A lot of roses that get used as "climbers" are just very large upright shrubs and others are too supple to really grow without support. I think for growing two together you would need to have one that is more sturdy and upright and one that is more lax and flexible to wind into the upright one. The problem is they'd have to be otherwise pretty compatible in likes and dislikes and pruning might become a nightmare as they mature. Would be pretty though, I'll bet!

As for your second question, if it really wants to climb and you keep whacking it back eventually it will probably become very unhappy and unhealthy. Even with standard size roses some just want to be big girls and others are slow to grow. Trying to keep the more vigorous ones smaller rarely works and they end up resenting the pruning and not blooming.

Here's the opposite: I took two canes on Duchess de Brabant and tied them to the outside two stakes in the picture. The growth on the top is too tall for the deer, so I will get some blooms. The rest of the bush is sprawling and anything I get from those canes will be a bonus.

I believe that the original Kiftsgate rose was planted many years ago as an unsupported shrub. It didn't mind, it just took over half a border.
Daisy

Here is a link that might be useful: The original Kiftsgate