When it comes to a FB-hoops double, GB is correct. It IS rare, perhaps as rare as that Holy Cross two-way star about ten or fifteen years ago (I forget his name). However, a FB-baseball or especially FB-track double is easily doable given the proper talent level. In fact, it's easier on the college level than on the pro level. Bo Jackson was an incredible FB-baseball double, and Herschel Walker was incredible in doing the FB-track double.
Of couse, we hear more about pro FB-baseball doubles because track at the pro level is almost nonexistent here in the US. Neon Deion was the last I remember, and he even pulled off games in different cities on the same day as I recall. He had to essentially cheese out of his baseball game to play his FB game that night, but he did pull it off.