I can't say conclusively that this will work 100% of the time, but I have had 100% success with first feeding a smaller live prey, and following it up immedietely behind with F/T. The snake will simply continue eating the second animal providing you get it in there before they close their mouth around the end of the first.
Just make sure that the total weight of both animals is an acceptable ratio for the snakes overall proportions. You don't want to over feed.
After the first time they should accept F/T on the next feeding. At least I have not experienced a refusal of the F/T. But I guess if I had, I would just try it again. (Rinse and repeat) This can also be applied to the switch from mice to rats.
As many people here have said though, every snake is an individual.