just don't let the chassis touch each other, then you don't have to modify their wiring. for best results, the power supplies should be as identical as possible.
as Nigel's pointed out, your motors may have much greater current demands at startup and if they stall. does your controller support soft-start of the motors? having a massive current draw hit those switchers will probably trip some internal safety regulator and shut it down.
oh, don't forget to give each switcher a minimum load on each of the rails - typically 1-2 amps.