From what I think you are saying, the answer is no. You can't feed on channel (L or R) to the amps (and then onto the speakers) and the other channel to just the sub.
You have to keep the channels isolated from eachother, left channel going to the left speaker and the right channel going to the right speaker.
This also brings up another question. You already said the sub doesn't have speaker wire connections (L and R) on it. Does it have a separate L and R RCA inputs? If it doesn't, then there is another problem. You can't merge the L/R channel signals back together without isolating them. Otherwise, you are creating a short circuit and loose the "stereo". Typically this is done inside the preamp/processor or inside the sub.
I know that this is getting complicated and confusing for you. Try drawing this out on a sheet of paper, it might help. And trying to explain this in text can add to the confusion. If you would like to talk about it, private message me and maybe we can arrange something. Maybe some kind of white board session
- Jeff