The delivery of a drug is dependent on several different types of enzymes and or transport proteins. In the particulars you're asking, berberine inhibits two specific transport proteins---OCT1 and OCT2 (organic cation transporter 1 and 2). Given that, metformin is dependent on the proper function of these two organic cation transporters.
Berberine will basically cancel out most, if not all of metformins action.
So, no not a good idea.