Both trees are F. benjamina. 'Midnight' is just a sport of the species plant.

People who are used to using heavy soils will tell you that you need to be careful about over-potting, and you should move up in pot size gradually. When it comes to trees, and plants with roots similar to tree roots, you really need to be thinking about tending to the roots, as well. Once the roots of a plant become congested, growth will always be inhibited, no matter how large a pot you pot up into, unless/until you correct the root issues by selective pruning (of the roots).

I wrote this about container size relations over on thew container forum several years ago. It explains the relationship between plant material (size), pot size, and soil choice:

"Over-potting" is a term that arises from a lack of basic knowledge about the 3-way relationship between plant material/ soil composition/ container size, which together logically determine appropriate container size(s).
It's often parroted that you should only move up 1 size in containers when "potting-up". The reasoning is that the soil will remain wet too long and cause root rot issues. Of course, that's true if you're growing in a heavy, inappropriate soil, but it can be remedied by changing the soil to one that drains freely.

The size/mass of the material and soil type/composition determine both the upper & lower limits of appropriate container size. Plants grown in slow soils need to be grown in containers with smaller soil volumes so that the plant can use water quickly, allowing air to return to the soil. This (smaller soil volumes) will cause plants to both extend branches and gain o/a mass much more slowly. Rampant growth can be had by growing in very large containers and in very fast soils where frequent watering and fertilizing is required - so it's not that trees necessarily rebel at being potted into very large containers per se, but rather, they rebel at being potted into very large containers with a soil that is too slow.

We know that there is an inverse relationship between soil particle size and the height of the PWT in containers. As particle size increases, the height of the PWT decreases, until at about a particle size of just under 1/8 inch, soils will no longer hold perched water. If there is no perched water, the soil is ALWAYS well aerated, even when the soil is at container capacity (saturated).

If you wish to grow in a large container - please do. Just be certain that your soil supports no, or very little perched water. You have all the tools you need to make an excellent soil. Now all you need is the confidence and the materials. ;o)

The short answer to your question is: Whether or not it is appropriate to combine the two plants and plant them in the considerably larger container depends entirely on your choice of soil. It is probably inappropriately large if you are using a slow/heavy soil, but perfectly suitable if your soil drains freely and is well-aerated.

Al

Al,as always I thank you for responding.
Since you are GW's expert on Ficus' is your answer yes or no please lol?
If it helps I use Miricle Grow brand soil mix.

I would discourage you from doing it now. Reasons - you should probably attend to the roots and do a full repot (includes root work) instead of just potting up when you DO repot, and now isn't a good time to do major pruning above or below the soil line. Also, if you don't bare-root, you'll likely have 3 different soil types in the same container, and two kinds is already one too many.

If it was my planting, I would make sure I have a good soil ready to go and I'd do it in late June. If you drive up to Bay City, we can do it together - make a day of it, and I'll introduce you to the gardens and bonsai. I'll supply the soil. ;o)

Al (I was serious)

I'll take your advice Al and hold off. You're going to have to give me a raincheck on coming to Bay City. I'm in the process of moving(or will be in the next few months). I'm not sure if I'm staying in the Metro area or not. If I come to Bay City I'm likely to leave your place and head to the nearest realtor's office&never come home again. Then were would my poor plants be lol?
As always,I thank you Al!