2 Bs will not be a deal breaker for you in medical school by any stretch. I don't think there is a need for you to take upper level bio to "make up for it" - only take the classes if you want to. A much more direct way to show your strength in intro biology would be to do well on MCAT later down the road.
Concentrate on your major, take classes that you want to (don't restrict yourself to science or you'll go crazy!!), and don't forget to have a little bit of fun along the way (don't worry I won't tell anyone)!
Hope this helps!