well first off, a passive crossover on just any speaker is kind of like a sub in a box thats made for a different sub.
so if you want to add another set you either need to get another amp, wire something to the HU, or find out if your amp is tri-mode capable.
as for the RCAs, you just use splitters if you add a new amp. make sure to maintain left and right channels. its all easy stuff. you should know how to do it.