You do not state how many channels you have on the RX, you need one for each servo that can then be set up individually and assigned to the same switch. If you have to use a Y lead you can get a servo reverser that will plug into one of the servos in the Y lead ,how ever what ever adjustment you make to one servo will effect the other, a better option if you do not have enough spare channels is to get a JR "Match box" this devise looks like a small Rx has one wire that is plugged into the Rx channel and four outlets for 4 servos that can be setup via the match box to do most anything you need .
Problem as you know is JR are no longer trading as such how ever I believe Futaba do one which may work? or some one may have a Jr one for sale.
Best option is to get a Rx with enough channels for ever servo in the model, which can then be set up individually end point etc and then assigned to what ever switch you require .
It is some times a good idea to copy the whole model to another memory in the TX so that when you are programing new inputs and get it wrong as I have been known to do, you will not mess up all the original settings.