Epl, what are you trying to achieve? Perhaps, growing one trunk so it's shaped tree-like?
10 plants in a 6" pot is 'multi' all right..How tall are each plant? Any roots growing through drainage holes?

It can be done, but you'll probably have to cut into roots to divide. I'm thinking, roots are tangles to high-Heaven.
Fiddling with roots can be risky.
Gardnias are difficult as it is,so after untangling roots, well, it's a 50-50 chance..Good luck, Toni

Don't forget to fall to your knees, wring your hands and through your tears, plead with your little gardenia cuttings to live for you. That's what I do with my plants...so far it's working.