What I'm getting out of it is that he has a Headunit with only two RCA outputs, a left and a right, AKA one pair.
He also has a substage amplifier and wishes to add an amp for his high speakers.
First things first, if you're going to power BOTH your front and rear speakers off this one 2 channel amp (a pair of left speakers and a pair of right speakers) you will lose the ability to fade the system front and rear. So your rears may overpower your front speakers.
I would suggest you leave the rears running off the head unit as courtesy speakers for any backseat passengers you may have and only amplify the front speakers. If you get the itch to upgrade your speakers later, I'd leave the rears however they are and focus on the fronts solely.
I havn't rand a rear stage in a car on at least 2 years and have no intentions of running any for a long time.
I can talk more on this later if you wish, but for now, back to the meat and potatoes of your question.
You have a couple options on what you can do depending on what amplifier you get. A lot of amplifiers come with a set of RCA outs along with RCA ins, so if you get an amp with a set of outs you can just run your first set of RCA's from the head unit to the new amp, then run another set of RCAs from the preouts on the new amp to the sub amp you already have.
If the new amp you get does not have a set of RCA outs then you'll need to get 2 RCA splitters and then run to each separate amp.
Personally, if you're already going to be buying a 2 channel amp anyway I'd make sure I got one with a set of preouts...