Tonight I had 2 beers. It was two hours ago that I had my last one and I've also had two bottles of water. Can I safely take my mucinex DM?

Theoretically yes, no interaction between beer and dextromethorphan, the cough sedating component of the Mucinex DM, but if two beers make you drowsy, and the dextromethorphan has this potential too, so it won't be advisable. Good that you asked, and good that you drank lot of plain water to flush your system, good luck.