Now would be a good time to separate them. Key limes have a harder time in lower temperature and both need more sunlight than many other citrus trees. Give preference to the meyer lemon tree over the key lime tree. It is probable that you will get more production out of the meyer lemon. Both trees will need supplemental light though the fall and winter to keep the trees healthy.

After separating the plant give the plant a couple of day rest from direct sunlight. Then place the trees in broken sunlight.

I just transplanted my poncirus trifoliata to a larger container and placed it in broken sunlight and it appears to be doing well.

You might want to read up on root pruning (broken roots). Click on link

Steve

Here is a link that might be useful: http://forums.gardenweb.com/forums/load/citrus/msg082311393319.html?15