6 speakers to a 4 speaker head unit?

Ok, i know all the standard reasons NOT to do this (wire an additional pair of speakers into the HU) in the traditional way, but after a bit of googling i havent found anyone mentioning the following: can i take the amp from an unused HU, wire it to power and use it to power the two additional speakers? I mean, IN MY HEAD this seems a reasonable thing to attempt ….

I just dont know the fundamentals well enough to know of potential pitfalls BEFORE my car lights on fire…