i don't think u can (simultaniously at least) since they r 2 different surfaces. ur best bet is to attach the 2 adjacent surfaces into one, or if u donot want to , u can do a half fillet on one surface then do another half fillet on the other adjacent surface ( i could be wrong tho... )
try also duplicating the adjacent surfaces the attach them (the duplicates) into one. (leave the dup. surface dead smack on the original 2 adjaccent ones, then with thte newly attached duplicated surface, do a fillet for it. then simply hide the duplicated surface this way only the fillet and the fillet object is visible along with the 2 adjacent original surfaces. hope this helped.