It is normal. Buns will mount each other for two reasons. One is hormone driven, the desire to mate and the other is to work out the pecking order, so who is the boss bun.
To truly have them bond you would want both o be spayed as well as to prevent cancer such as uterine. Your buns may figure out the pecking order now and get along if the 12 week old girl is not yet hormonal but once the hormones kick in they could fight. Rabbit fights can be bad.
Having them spayed will stop mounting for hormonal reasons and will make it so they can truly bond. That being said after the spay there could be some mounting to decide who's the boss.