I let mine get more than one stem. It's difficult to predict how much of a crop you will get from a plant because so many variables are involved - fertilizer, temperature, etc.

That being said, you definitely will get more tomatoes from two stems than one. I have heard that you will get larger tomatoes from a single-stemmed plant, but I can't speak from personal experience.

Here is a link that might be useful: Tomatoes on SFG trellis