I was in a similar situation. I had a year and a half to graduate with only one requirement left on my first major, eyeing a major that I had already made some progress towards and that I liked more. I can't say what you should do, but I chose to double up, and I was happy with the result.
It won't make any difference to anyone else but you. It probably won't make you any more competitive than if you just took the bio courses without adding the double major. If you think it will impact your GPA significantly, then take the route that spares your GPA. Good luck with your decision.
Edit- it wasn't that clear from your original post, but if doubling would add another year, then definately don't do it.