Mainz and Heidelberg in one day?

I've one full day to spare while based in Frankfurt and am considering going to Mainz &/or Heidelberg. Is it possible to visit both places in a day? If so, should I buy separate train tickets for each leg of the journey or is there some kind of day ticket I can purchase? I'll be going on a Sunday if it makes any difference.

I don't know about your train tickets query but think that's no problem: visiting both towns in one day is a bad idea if you want to really visit them and not only will scratch the surface..

Like Arie suggests, you'll be doing yourself a favour by just choosing one of them...

If you really want to go to both places in one day, the Happy Weekend ticket will be valid all day Sunday until 3 am Monday morning, but on local trains only. Check trains and times on www.bahn.de and make sure to click on "local transport only".