That means trunk to trunk, your trees are only about 3 feet apart? With M27 you may be OK, but that seems pretty close, and I'm not familiar with potted trees. Others
probably more knowledgeable about this, but I'm concerned that your going to need to spread the trees farther apart. I think you'll be ok with removing 1/3 of branch renewal,
but you don't want to get locked into a 'vegetative cycle', where you remove so much
that the tree puts its effort into growing more vegetation at the expense of fruiting.
Also remember that different varieties grow with different vigor on the same rootstock.
Chris Mauchline SE PA, zone 6 40° 5' N 75° 51'W ~650 ft elevation
<<My 2 foor diameter pots are about 1 foot apart. I took Tom's advice and trimmed the lower scaffold branches back by 1/3. I am training the central leader up a stack in an S-fashion. My hope is that by controlling the leader in this fashion, I will promote lateral branch and fruit development. My trees are only in their second year but I did get a few flowers. While it might be best for me to take these off, I canot resist keeping them on since I am inpatient for my first apples.>>