Due to what the others have said, there's a reason for why they invented the 4-channel amplifier.
The frequency response, impedence and power levels of the speakers are all different. If you run the setup you want to run on a 2-channel amplifier, you run the risk of over-driving the amplifier as well.
The op amps inside the amplifier (Basically the transistors that increase the current to produce a larger audio signal) are going to be run harder to give a larger audio signal to all the speakers. In the short term, its okay for testing purposes perhaps but over time, you're going to diminish their ability to reproduce the same audio signal. In some cases their won't be any noticeable run-down, they'll explode if the fuse doesn't explode first. You're just adding more weight to a distance runner metephorically.
You get this a lot with some circuits that contain darlington pair transistors or op amp drivers which are a setup to increase the current in the circuit in order to increase the power of a reference signal. You find this on tranceiver radio systems as well every time you transmit, transistor setups like darlington pairs / op amps will drive as hard as they can to produce the audio signal being pushed out of the antenna. So long as you keep transmitting in bursts, you'll be fine. Prolonged transmissions will deteriorate these transistors and they will blow if you don't take it easy with the radio. Going back to the 2-channel amplifier, essentially you are doing the same thing. You don't run the speakers at max all the time but adding more speakers is essentially asking for more from the amplifier as well. Keep the tolerances within specification and you'll be fine.
Save a couple hundred bucks and get a decent 4-channel amplifier. It is worth the stretch mate.
