look closely at the crossovers and remote options on the 2 channel amp
the 4 channel doesn't have that, to my ears and others it sounds better enough to mention it as a "want to have".
you can hookup multiple speakers to a single channel, and drive down the ohms to a 2ohm load, best performance v. dollar
not sure which cadence, some are listed as 4ohm, some listed as 3 ohm, which is 2ohm by application
if your total speaker pairing(s) provide a 2ohm load you can draw 170w-190w from that single channel, enough to drive both speakers.
have about 600-800w, use about 400-500w, that is your cleanest.
the head unit will do most of the work, agreed, but you should reinforce good crossover points, by duplicating them on the source, amp and speakers.