Are you trying to "strap" them together? If so it can't be done, amps must be same and be strapable.
Hifonics has at item that allows you to strap amps that, under normal circumstances, would never be possible.
If you mean bridge as regular ol' bridge of wiresing, that a run of + from one channel and a run of - from the 2nd channel.....Don't use both amps to power subs by bridging each to the subs either.
You only really want one sub amp to power your subs as they are all different. Rms will be different and so on. But if you mean have one amp set to the correct voltage to power ALL subs, then you are correct.
Mixing and matching only work if say you have one amp powering subs and one amp powering speakers, then you are safe.