We have never been able to combine the Disney Visa OBC with any other Disney provided OBC, other than the $25 OBC for booking online. The Disney Visa OBC is a booking code discount (code is DCC) and
DCL only allows one booking code discount per reservation. So you can either get onboard booking benefits, Disney Visa benefits, kids sail free, military, etc, but not more than one since those are all booking code discounts. For this cruise, we are getting the $25 online OBC, $50 Disney Visa OBC, and the OBCs from our TA. (Despite sailing 2 weeks after booking, we were never able to rebook our original opening day reservation for this cruise since it had already sold out above our category by that point.)
The OBC you receive when you sail on an onboard booking is based on your CC level and the length of your actual cruise when you sail. The benefits freely transfer with your reservation as long as you don't cancel it, though there are some sailings (like the Fantasy MV) where you don't get the full onboard benefits (OBC, but no discount).