It is likely these are all suckers growing from the roots of a nearby tree. They are joined only by the original horizontal root but they have been growing long enough to develop their own roots from the new trunks. That is usually the case with sweet cherry.
The plate may hold then together long enough for the roots to entwine. They may even fuse together.
I find the growth habit and long internodes of these cherries make bonsai styling difficult not to mention pest and disease problems so good luck with developing these as bonsai