Ozzy's on the right track.
To control multiple engines at once, press "E" followed by the number of the engines you want to control
For example, if you want to control only engines 1 & 4 on a B747 (can't think why you'd want to

), you would press "E14".
So to control all four engines you would press "E1234". Make sure you use the number keys above QWERTY, no the numerick keypad.
Hope that helps.
Jock in a Frock