Different Gene Combinations...

[sassy-pants]

Is it possible to put both males with the females and get a mix of both genes in the babies?

Yes. Don't put the males in at the same time, cycle them through a few times over your designated "breeding" period.

Also is Rex a recessive trait or what does it fall under?

It is like a co-dom. A "double rex" often results in a mostly or completely hairless rat. A rex is just a curly-furred rat.