There are two ways to connect a pair of speakers
together. You can have series or parallel. Easy
enough?

What happens when
you hang that twin speaker extension cab off your 1
x 12"?? Hmm... gets a bit more complicated. I've
put together an Excel 5 spreadsheet which caters for
many different wiring combinations of 2, 3, and 4
speakers.

Given the power
output of the amp, it will work out the total
impedance that the amp "sees", and will work out the
individual power within each speaker. It has
pictures of the different configurations within the
spreadsheet, so you can use these as a reference for
wiring the speakers up too....