I agree with all Kevin has said above.
Many people will say keep phases seperate, however you couldn't get phases much closer than in 3-phase cable which is just as likely to have a fault as 2 lanterns being fed from different phases, so it's a little pointless on a modern RCD protected system!
Also, many large 3-phase dimmers with multi-pin outputs (Socapex) will be wired channels 1 to 6 phases 123123, or 112233, so again it is expected you'll have multiple phases on a bar.
Of course when you can and it is reasonably practicable, then it is best practice to seperate out each bar.
For your scenario I'd probably put one phase Betapack for FOH, and the other pair of Betapacks on another phase for over stage bars. As long as the electrical installation is RCD protected and regularly tested you're fine.
Edward