I have two littermates (male and female), and right from the word go they were crated seperately
Did not want to get into the problem of them becoming overbonded to eachother. At training we sometimes get people with dogs who are far too attached and it is a huge problem to overcome and involes an enourmous amount of work.
I actually don't like crating dogs together - 99.9% of the time it might be okay, but the risk of the .01% fight is not worth it to me. Have seen a fight happen in a crate and it was not pretty