I will assume you've done this with embedded roots before, and know the proper technique for working with something completely rootbound so you don't kill the plants? 'breaking up the roots' is a very bad idea without knowing exactly how fragile they are as you start to work through the entangled mess (see: migraine and 4-6 hours of time).. and then you get to the fun part of root pruning (which is a whole other subject, and I could write a 30 page book on).. after all that, you then hope you didn't over-do it, and stunt growth or murder them. Easier to start them off separate, unless you just can't help it.
Just my .02,
Fi-