I would argue it depends on the variety. For example, if I had a small low growing variety like China Doll (18" tall, 4" diameter flowers) I think it would be fine to have multiple main stems. On the other hand, if its a 4+ foot variety or one with large flowers, I wouldn't want two that close together as they'd be competing for the light/water and lots of flowers would end up being mushed between them.

Of course your blooms may be a bit smaller because of the competition, but it won't really matter unless you're showing.

Cheers,
Russ