Can I amp front and rear speakers and a sub with a HU with two RCA preouts?

Yes, you can. Run a pair of RCA cables from your deck to your sub amp and make sure the amp is set for subs (LPF, LowFR, Sub, different amps call it different things.) Next run the other RCAs to your fronts/rear's amp and set it for normal use. Check the RCA outputs for labels as they usually have a dedicated pair for subs and one for regular frequency use.