Is it safe to mix breastmilk with regular formula milk?

Okay..say I have 2oz. Of formula milk already in the fridge...and I also have 1 oz of. breastmilk left that I pumped in the fridge...can I mix those two together in the same bottle and heat up and feed it to my newborn?? Or do I have to heat them up desperately and have her drink them seperately?? I know it's a stupid question but I'm new from and I don't want to make my baby sick or anything.