Interesting concept... it's all marketing; once it sells, they don't really care what happens after that. I don't think any reputable grower would sell you those two in one pot. Aside from cross pollination, which is not a major concern, the Key Lime might be overfed when the Meyer at the same time would be underfed.

I would not transplant them when they have fruit; but if you thin the fruit first, then wait a week or two, then you are VERY careful with the roots and add a very small amount of food, it might work. The danger is that if everything is not perfect, the tree(s) will drop leaves to save the fruit. If your Brother is an accomplished container grower, he can transplant them later; you are so right about double shock. Some people actually triple or quadruple shock their trees, transplanting them, moving them, changing light/temperature/humidity... and STILL the citrus survives. For that citrus is an excellent plant for container growers; they can often survive even the cruelest/most "inhumane" treatment.

Humm...I think you may be right, I was at the garden center a few weeks ago and my citruses actually looked better than theirs. Since then I've brought them inside and they've lost some leaves due to the adjustment. Plus my existing citruses have buggie issues (scale, aphids, and mites) which I treat with cotton swab treated with alcohol as well as occasional Neem sprays. When they came indoors the bugs went nuts so I've done a major cleaning but there has been some leaf drop. They are covered with blossoms though. They are also kept away from the plant for my brother.

Your comments about the different feeding probably explains the Key Lime being smaller than the Meyer. I figure I'll do the transplant and hold onto it until it's ready to move. My brother's place will have a nice sun room, no bug infected neighbors, and he's local so I can/will provide any major care. I just wish I could get rid of the bugs once and for all. They came with an ornamental tree I took over from a friend who moved over 8 years ago. Alcohol is my friend...

I've just finished answering another question about cross-pollination when I saw this similar post. Take a look at "Hybrid Fruit".
Cross pollination doesn't affect the fruit in any way, except to produce seeds which may themselves grow on to produce hybrids. The lemon will always produce lemons and the lime always produce limes. The fruits are part of the parent species and can't change - the offspring are the plants grown from the seeds, and these may have hybrid fruits.

Yikes, I have already found more helpful information here than at the alleged grower's website. I also have a tree/plant identified as a Meyer Lemon and Key Lime. Do I separate them or just replant to a larger pot? The Key Lime tree/limb has plenty of flowers but I do not want to jeopardize either by delaying necessary action.
Just because I did not kill the tomato plants as usual, I began to think I am a farmer!
Any information would be appreciated.
Jan