Have an Alpine cda-9825 head unit that is 18 W RMS × 4.
Only have places to put 2 speakers and have a pair of 6x9 Pioneers available RMS Power Range (Watts) 2-80 Peak Power Handling (Watts) 350
Will need to be played relatively loud as it's a loud old truck ('78 International Scout) .. and speaker location isn't the greatest.
Does a 4 channel head unit running only 2 speakers give any extra boost to the 2 speakers ... is there any special wiring needed or just leave the fronts taped off?
Will the head unit drive the speakers ok, or is an amp needed? What would be a decent low-power amp ... low amp alternator and anything more would require complete rewire.
Anything powered with an amp is generally going to sound better than being powered off a cheap headunit amp. If you want more volume you could go with a component set, all depending how much money you want to spend. The Infinity Perfect 6.1 set is pretty decent, and the Infinity coaxials are good for the price. You should be able to run any of them with a 100x2 amp or a 150x2 amp.