it's OK to play a professional baseball game with only one umpire. I went to a day game of our local single A team today. After an inning and a half one of the umpires had to pack it in due to heat, so the game was played with the plate umpire running out from home when the ball was hit to make calls on the bases. The potential for disaster in this situation seems big - what if there's a ball the ump must call fair or foul in the outfield and a base runner misses third?
In fact the game wasn't close enough for any single call to matter (we lost 12-5), but know I'm glad that childhood interest in being an ump never worked out.