Real answer? no.
Seriously, I think the real question is "why would you?" Now, a thoughtful and ambitous med student might tell you about fund of knowledge and broadening one's mind, which are both things that I value; however you'll find in the practice of what you do, you will not likely be much better at handling what you're going to do for you patients as a dual boarded doc; one gets "rusty" if one doenst practice medicine; so while it may be great that you've done lots of, oh lets say, pericentesis as an IM, you're not going to be doing them in rad onc-not cos you can't but simply becasse they wont come up on your watch and the rare times they do, you'll be too rusty-Ive done plenty in my intern year but I know that since I've not had to do them for 4 years, if one comes up next week I wont do it (nor, by the way, would I want to- another nice thing about staying in your field of interest).
The common things you'll need to deal with (medicines, cardiac, respiratory issues, DVT, PEs, etc) you'll see as a rad onc doc and you certainly don't need to have an internists degree for it.
Case in point is my current attending who did IM and med onc first before seeing the light and coming over to the side of the Beam in rad onc: he does wright chemo orders occasionally, but even his patients see med oncs who practice med onc full time; he doesn't have an in patient service (rad onc doesnt generally) and so really, the stuff he's doing/perscribing day to day is merely the stuff he would be doing/perscribing as a rad onc doc anyway. AS his resident right now in fact, I'm the one doing most of this stuff. (Managing the whipple related diabetes, giving pain meds, lasix etc) and I didn't do more than my one year IM internship.
steph
