just wondering can you run two 50 watt mercury vapor lamps on a 100 watt metal halide ballast.

You maybe able to but i would not because i don't want to take a chance at losing a ballast!

just wondering can you run two 50 watt mercury vapor lamps on a 100 watt metal halide ballast.

No, if you want both lamps as well as ballast survive it.

How would you wire that? In series or some other way? Because you only have one wire for one bulb or is it made for two bulbs?

well, if you wire them in series, the voltage drop is too much for the ballast. So the ballast will likely overheat. If you wire them in parallel, on the other hand, there's nothing to stop all the current from going to just one lamp. So only one will light, and then be driven at a full 100W until it blows.

So, as stated above, not a good idea.