Two possible solutions (just to warn you, for the first one you will have to delete your save file; also, this might take a week or so):
Solution 1
1.) Receive either one in HG/SS
2.) Migrate it over to B/W
3.) Delete the save file on HG/SS and restart, getting to the point where you receive a Hoenn starter again
4.) Migrate the other one over to B/W
Solution 2 (you must have R/S/E)
1.) Follow steps 1-2 for Solution 1
2.) Migrate a Hoenn starter from R/S/E to HG/SS to B/W
Or just use the GTS.
Slightly complicated, but at least they work. (-o-)