Can you convert speaker wire from HU to RCA' that will transmit highs?

My buddy's head unit only has 1 pair of outputs for RCA cables, which are connected to a Hifonics 2000D amp and his 2 12" RE SX subs. He's looking for better highs and wants to add another amp. So, is there a way for me to convert some RCA cables that will be suitable for a 4 channel amp? I'm worried If I use a high to low filter converter it wont give me the right frequency range i need. Thanks.