Hi. Having the same signal go to the subs won't affect how loud they play unless the splitter has a lot of signal loss - which is unlikely. The amps in the subs are the important thing - and just to verify, you are getting powered subs, correct?
If you have powered subs, then an RCA connection is just fine. Whoever said that RCA cables don't carry 20-250Hz was wrong. Subs, which are intended to play almost solely in that frequency range, wouldn't commonly have RCA connections if that were the case.
I connect my subs using RCA connections, and they work perfectly.