Is there any particular reason you want to use a slider to control throttle? I mean, that throttle lever (w/trim) is just sitting there waiting to be fondled.
Assign both engine servos to the throttle control (just like you would assign two different elevator halves to the same elev. control). Be sure to set the servos to have 5P adjustment capability.
Then adjust the full open and full closed throttle limits in the the Servo Calibration menu to be close to each other.
Then you can fine-tune the servos with the engines running by adjusting each of the 5P points which would allow you to adjust the two engines to be in sync throughout the entire power band.
Naturally, the trim would work with both engines.
If you MUST have the capability of adjusting an engine separately while flying (but why, when you can fine tune the engines by adjusting the 5P curves and be done with it?), then go ahead and assign a slider to one of the engines (you'll then see that BOTH the throttle AND the slider will operate the assigned engine). But you'll be futzing with it all the time when you "should" be flying the aircraft. Naturally, if you want to make the actual engine sync adjustments in flight, you would simply assign one of the digiadjusters to one of the engines and have at it.
Is that what you were looking for?
Highflight