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.

do you know any good examples of complimentary ideas?

in terms of unrelated ideas: Max Levchin started Glow and Affirm almost at the same time, Elon runs two companies.

additionally, at the lower level, programming patterns are almost the same.

I was talking about early stage entrepreneurs. Working on multiple ideas once you are successful or have enough experience sounds logical but initially that might be a hurdle.

Check this article for examples. Really good one http://ryanhoover.me/post/98567029388/startup-side-projects