Do you need a second sub….maybe/maybe not.  A lot depends on your room acoustics and the potential for standing sound waves.  Adding a second sub can fix that with parallel walls.  It gets a bit technical and you will have to take some room measurements…so I would do some reading on standing sound waves.  

Using the high level speaker inputs on the sub is a preferred method by most here on the forum.  My setup is a bit different only because of my room setup and the optimal placement for my sub.  In my system I utilize the Decware ZSB switch box which has 4 inputs and 2 pairs of RCA outputs.  When I was building my house, I had run a pair of RCA cables in the wall. One set of the RCA outputs go to my CSP preamp, and the second pair are connected to the sub which is located on a side wall.  

Hope this helps.

I don’t use subwoofers but I’ve seen this question before. If the amps are bridged but not balanced, and you want to use the high level inputs to the subs I think you need two. Then you can connect the plus and minus outputs just like you would for a stereo amp. Trying to connect outputs from two mono blocs to the same sub is asking for hum. If you want to stick with a single sub you probably want to use the preamp output.