I can only offer general advice, as I'm not familiar with your specific transmitter, but maybe someone else who is will step in.
First general advice. Never experiment with motor settings and mixes with the model plugged in! Do all your setup work using the transmitter's display screen. It will tell you everything you need to know without the risk of motors starting!
Second, the issue with the slaved second throttle channel only starting at mid throttle is a matter of mix settings. Maybe you only set the mix for the up side of trhe curve (there are normally two numbers that must be set, like +100% and -100%). If that doesn't do it, there should be an offset number. Try -100%. Look at the results on the display screen, which is far more informative than whether the motor starts!
Third, once you've got the two throttle channels working in unison, which should be very easy, you will need to look at the steering mix. The first thing to recognize is that the mixes you want are from rudder to the two throttles, not throttle to rudder as you describe it.
The following may give you some ideas as to how the various mixes are set up in general terms. It's not about Futaba but the principles are the same.
Note that the key term for searching is "differential throttle".
Hope this helps.
EDIT: Here's what Futaba says: