With most any computer radio:
Slave another channel to the throttle channel (I use Ch8, Link on, Trim On)
Make a P-Mix, Master = throttle channel 2, slave same. Assign to a knob or slider. Adjusting the knob/slider will vary the speed of the second engine.
Futaba has a FAQ that may help also:
http://www.futabarc.com/faq/faq-9c-q511.html
Fritz