ok, so i used to have a twin sub bridged to a 400wrms 4ch amp. i'm planning on replacing those with a 1500w sub.
I was thinking i'd just use the other 2 channels to power the 6 inch splits i got in the back

Is it recommended to have 2 channels bridged and not the other 2?

I think you will find this is ok. My amp actually has instructions to run a sub bridged accross 2 channels then use the ssame channels to run 2 more speakers.

with a 4ch amp you can bridge the amp to make it 2ch
but you can also have 2channels bridged and the other 2 for speakers
normally channel 3 and 4 get bridged

sweet

I think you will find this is ok. My amp actually has instructions to run a sub bridged accross 2 channels then use the ssame channels to run 2 more speakers.

Most manufacturers refer to this as tri-mode, they run bridged and stereo simultaneously.
Alpine MRV345 and 545 are common ones that do this well. The old Phoenix Gold M50 did it a long time back.

i'd recommend you go get a 400rms amp and run the subs on 2 ohms. would make one hell of a difference