If you have a 100 watt amp can you just have two 50 watt speakers instead of a 100 watt speaker?

Maybe.

The real question comes down to the resistance of the output on the amp vs. the resistance of the speakers. You'll have to do a little math and understand the difference between series and parallel wiring. Throw back to high school physics.

When you're wiring something in series the resistances are added. So, if you have two 8 ohm speakers the total resistance is 16 ohms. If the output resistance on the amp is 16 ohms, you're in good shape, but don't crank it or it definitely will blow.

When you wire a parallel circuit you use the formula 1/RT=(1/R1)+(1/R2). Again, if both speakers are 8 ohms then it's 1/RT=(1/8)+(1/8). With a little math you get 4 ohms. This is WAAAAAAAY different from 16 ohms. Turn that 16 ohm amp on now and those speakers will turn into a fun light show, but they'll be toasted. Throw some butter on it and you've got breakfast.