Nipples on male rabbits are, in my experience, really hard to notice.
Humping shouldn't be used to determine sex - both males and females will hump to show dominance.
The only sure way to tell what sex they are is to look at their genetils.
But either way - you shouldn't be putting them together or trying to bond them until they are fixed. If they are both the same sex, they could fight. If they are a male and a female, they will breed - it only takes seconds for the females to become pregnant. It wouldn't be a good idea to breed them when they're are already thousands of homeless rabbits in shelters. Also they are related, so that is considered imbreeding and can result in some very unhealthy babies.
http://www.debmark.com/rabbits/sexing.htm