Without details of the equipment you plan to use, it's difficult to suggest anything too specific.
Lets assume the amp is a normal hi fi, 8 ohms, which accepts one pair of speakers only. As an example, two eight ohm speakers (and most are these days) wired in parallel is 4 ohms. You'd get more volume, but really risk damaging the amp and speakers due to the larger amount of current passing through them.
The same 2 speakers in series would be 16 ohms . The formula is: 1/R1 + 1/R2 = 1/RT .
As a general rule, don't lower the load impedence by wiring multiple speakers in parallel to the amp, as if the impedence of the load decreases enough, the amp would 'view ' it as almost a short! OTOH, Wiring speakers in series increases the impedance (in ohms) of the load, and decreases the current through the speakers. Unfortunately it also decreases the amount of output audio power in most cases, since power is inversely proportional to impedance for a given voltage. So heads you win tails you lose kind of situation.
the ideal solution would as I said look for an AV amp which can already handle various sets of speakers safely. Buy the best cable you can (as a minimun, 2 core mains cable thickness) to run them to the two rooms, so you don't lose too much signal.
hope this helps!