I have one of those radios too and find that it is needlessly complicated and quite often does not do simply what seems to me to be a simple request. Instead, it's a big deal.

My solution is to just call Futabe and ask them how to do it. They make it even more complicated. Ususally, they phone back a day or so later and announce that they've figured it out and it requires some even more unnecessarily complex mix. Mind you, that usually gets the job done, if you can figure out what they are saying.

From what I gather, you are simply trying to map the throttle channel to a specific switch and then link a second channel to your throttle channel and have it mapped to the same switch so that you get the identical response to your throttle commands from both servos and have individual trims and end point adjustments on the serovs. That seems straight forward enough to me. But this is a Futaba radio.

I suppose an easier way would to be to use that little accessory box they have into which you can plug up to 4 servos and individually adjust throw direction, end points and trims. Then you would need only one channel and everything would be simple. Maybe. I've got one of those boxes for use with 4 aileron serovs but have yet to try it.