Two 8 ohm speakers can be wired as either 4 ohm or 16 ohm. That's it.
As to differences in sound, yes, they'll be different. You could put one 1x12 on each side of the stage to spread your sound out, you could put them next to each other angled apart to "fan out" the sound, there's a lot of ways to do it.
Comparing using two of the same speakers, spreading them out has some great advantages. If you have one speaker roughly behind you and the other roughly behind the bass player and he does the same, everyone in the band could hear one another better and you wouldn't have to fight to be heard as much. A lot more of the audience could the higher frequencies much better. You would also get much more of the stereo effect, meaning it would sound like one continuous, large speaker across the stage and could stand in more places and still hear yourself. You would also find that you wouldn't have to play quite as loud or need as much effect to have a full sound. It especially good with time based effects too.
There is always a difference with:
Open versus closed versus 'tuned' (semi-open)
Cab construction materials/design
speaker characteristics, et al
Steve Vai used to use large 4-12" boxes and then had 2 1-12'ers alongside the drummer's riser. It seemed to give him a continuous sweet spot. But that can also be attained with monitor EQ/placement.
Jus play around with your set-up and have fun. From stage to stage, club to auditorium...that's the challenge.