Can i take tussin dm with mucinex (guaifenesin) er?

No: There have been studies that show that honey works significantly better than cough / cold preparations. They do not work. My favorite thing to do for congestion or cough is simply saline to rinse out your sinuses, or the neil med sinus rinse which goes up and over to wash out your sinuses, but don't any of the otc cough/ cold stuff it just makes you worse.