I was wondering that too. Did you plant two different types or did you plant one later than the other? I would also try to thin it down, but by cutting because you are going to disrupt the roots. The only thing is, they will probably start to flop so I would experiment a little.
Also, I would throw down a little compost in between. If I remember right, cosmos will root along the stems so if you mound up just a little compost it should be OK.
When did you plant the seeds? We had a cold start this year. I have cosmos I planted in early June from seed. It was cupcake and they aren't blooming yet either. Sonata self seeders just started blooming. They are smaller. Ladybird cosmos which are the yellow/orange/reddish short type came into bloom pretty quickly.
