I would think from a strictly technical standpoint that if it was possible to diplex 1490 into the 1400 antenna (which from the last time I saw it consisted of a skirt wire system on the tower). I know WSHU-AM (the former WMMM 1260) is looking at co-locating to WFIF's site and diplexing into their towers. There's a much greater frequency difference there.
I don't recall what the required signal to the COL is but I would think it should be attainable since the physical difference between to two cities isn't that much. Of course, it's pure speculation on my part.