I'm currently in year 1 of experimenting growing bonchis (bonsai chillies) in both hydro (2x4 hydroton, flood/drain) and soil.
Part of this experiment has involved growing 2 or 3 in one pot (or next to each other in the hydro) and twisting the stems together so that they later 'fuse' into one stem.
So far, I have found that it's a lot harder than growing single plants. Even if I plant 2 4" seedlings of the same size together, one tends to dominate the other and kill it. I have given up on the fusing in the hydro and seperated them. In the soil, I am now trying growing them in seperate pots next to each other and then joining them above the pots.
Interestingly, I have never had a problem growing in the hydro table even though it's essentially one large pot, until I put the plants next to each other.