what is the rms power (not max) of the subs
and is the amp 600rms or 600 max
so it works ok with 1 sub but when ybsou connect 2 it goes into protect
if thats the case then your amp is not happy with the impedance (load on the amp )
what coils are the subs eg single 4 ohm coils dual 4 ohm coils