I think it varies from case to case. I wouldn't generalize it.There have been cases where things which started as a side project at a startup have become their main product.

At the same time there have been cases where startups didn't take off because of focusing on too many things.

So I think it's always better to work on complimentary ideas which might blend well together. Working on totally unrelated ideas at the same time would spoil the show. Again, I might be totally wrong too.