Jan, I've bred and raised several generations of bimacs. Other Eurasian ratsnake lovers have too. One breeder is trying to get an albino female to produce. I'm trying to get a couple color phases started. These are neat little snakes. You can take advantage of their fall breeding, or wait until spring and breed them like any other ratsnake. In fall I wait until they are off food to put the female and male together, then start to lower temps. They will usually mate before separating for brumation. You can leave them together all winter or separate. Put them together after their first shed. Try to get the male to shed first.
If you put them together while it's still winter, the only problem I can think of is that it might be too dry. I would try to get the humidity to at least 40%, keep a large water container in cage, and maybe a moist hide box.
Good luck with them...TC.