Just for information, this dude (? - maybe a chick, I don't know) posted a similar thread in the rads forum, and a few SDN radiologists took the opportunity to MF EM docs. One guy (again, ?) even came into this forum and posted something in the finance thread, and edited it out quite quickly.
That said...
As an EM resident, then attending, you will be looking at all of your own plain films, and you have to know how to interpret a head CT through a transverse plane that crosses the basal ganglia. You do NOT have to be able to interpret U/S (except the ones that you do bedside), MRI, chest or abdominal CT, or angiography.
So, being rads boarded, you will be put into some "interesting" (as in, "possibly risky") situations. First, you will see something on a plain film the radiologist will miss. Or, you will miss it, and the radiologist will find it, and that can range from ribbing (gentle or otherwise) to threats of litigation (even if you are not working as a radiologist, you are trained and boarded as a radiologist, so you could be held to the same standard). Likewise, you will order a certain modality, and the radiologist will refuse to do it, or you will speak with the radiologist, and rads will say 'X', and you will say 'Y', and that can engender a problem if you don't say "OK, 'X' it is", unless you can, diplomatically, make the case for 'Y'.
Your colleagues will put you in a difficult position by curbsiding you to look at their plain films (less risky) or CTs, which pulls you away from your patients, and can cause friction when you say it, and your colleague takes it and says it, and, when pressed as to why they say it, they'll refer back to you. However, if you decline to look at the films, there will be bad feeling.
Also, as an EM resident, you will do "radiology conference", where some of your residency mates will expect you to do the 'heavy lifting', and to know everything at all ever about radiology, and others will resent you, and will cackle with glee if you get something wrong or don't know it.
Those are just my first thoughts. As rads takes their boards during residency, I don't know if you can train in it without taking those exams.
As to actually doing it, I know of one guy peripherally, but I don't even remember his name.