Let me get this straight.
You have two subs and two amps. The two subs are rated at 400wrms. One amp is rated at 400wrms. The other amp is rated at 200wrms. Yes?
In that case, take the bigger amp, hook both subs up to it, and ignore the smaller amp for now. I hope it sounds good to you, but please consider a different brand of subwoofer for your next install..