BUT DON'T FILL THEM RIGHT UP!!!!!!! you park on a slope and the fuel will decant into the other tank and either come out the cap or the breather!!!!!!
You should never fill a Mini tank right up to the filler neck anyway but if you join both breathers together and tee the vent line off the middle of their conection you can reduce the chance of spiling fuel from it.
If you fill just from one side you'll be at the pump a while, they are only joined together by a 1/4" pipe and gravity takes a couple of minutes to balance them. As biggav said, splosh a fair bit in one side then go round and finish filling from the other.