Brumating won't keep them from mating while she is too young and even if you adjust the temperatures and amount of light, they may not brumate anyway. Also, they may not brumate for a full 4 months and even if they did, your female wouldn't be more than 10 months old and that is still way too young to breed her. There are alot of risks involved in breeding your dragons at too young an age. SHe could be highly stressed by the male always trying to breed her, the stress could leave her vulnerable to parasites and coccacia infections, cause her to lose weight etc. THe stress to her body for producing eggs can also run her down and a small dragon could go down hill alot faster than one who is older and with alot of energy reserves.
Most breeders recommend waiting till the female is at least 14 months old, 18 months or more being optimal. If you really care about the health of your female dragons and those of her offspring, separate them both now and wait another full year before breeding her. Also, a 40 gal breeder is rather small for two adult dragons, both should be housed individually in a 55 gal long (4'x2'x2') tank or larger.
Health concerns aside for your female, what about the babies? Are you prepared to incubate 20-30 eggs per clutch and a dragon can have as many as 5 or 6 clutches in one season for a potential of 100 eggs total? INcubation is just the first step in rearing baby dragons to an age where they can be found homes. They will eat thousands of crickets, need their own tanks (with up to 5 per tank to avoid over crowding and nipped toes and tails), vet checks and/or treatments, all the proper lighting for each cage etc? Also, if you don't sell all the babies, will you be willing to provide for these dragons till you do find homes or for the rest of their lives if homes cannot be found for them?
THink it cost nothing to feed these little ravenous babies? Just think this way. Each baby bearded dragon can eat as much as 50 1/4" crickets a day. Multiply that by a clutch of 20 babies and that's 1000 crickets per day. It would cost $15 per day to feed just 20 bearded dragons....and thats just the first clutch, expect 2 or more to hatch before that first batch is a month old. Then you are buying two sizes of crickets and greens or pellets etc. That $15 a day quickly adds up.
And to think, all this can be prevented with the expense of maybe $100 to build/buy a new tank for your female to live in alone.