Im getting a Mono amp to power a sub and the amp is going to have a crossover for the rca to just go through that amp.( I have 2 amps) or '' pass thru'' does the other amp need to have the blue remote wire too ?

yes they both need remote wires because they are remote turn on wires that tell the amp when to go on and off.

so just run 2 remote wires from the head unit to the amps ?

you can can just run the main wire to 1 amp and then a second wire from that amp to the other amp.