Hello, My name is XXXXX XXXXX I will be glad to assist you.
What you need to use is a pair of RCA Y splitter adaptors. At the connection of the subwoofer amp, simply unplug the RCA cables going to the amp, and plug them into the adaptors. Then the adaptors give you a total of (4) RCA ends which you will plug 2 into each amp.
Here is a link to the product I use for this same exact purpose:
You'd need two of them.
I cannot draw up a diagram.
It is super straight forward. You have two RCA ends right now (Left and Right).
Plug each one of these ends, into the Female end of the adaptor I linked to.
Then the adapter will give you (2) Male RCA ends. Plug one end into one plug on one amp, the other end into one plug on the other amp. Repeat for the other RCA cable.
Once you order the two cables you'll see exactly how they go it's very straightforward. They are what you need to connect two amps to a deck that only has one set of RCA outputs.