What you need is a powered crossover. It will separate the bass frequencies from your main channels and send them to your sub amp. I am providing a link to a specific example, but there are many out there.
I have two different amps. My smaller amp is powering my 4 speakers, and I would like my bigger amp to power my two subs. The problem is that my head unit does not have a preamp sub output. How can I wire this?
Check if your amps have a pass through RCA outputs. If neither of them have it, then you can always use 2 y-adapters to split the RCAs into 4 rca cables instead of 2.
