The mixes in a Hitec Eclipse don't include the trim function. You can create the mix to move both elevator servos with the elevator stick, but the trim tab will only move the servo that's plugged into the elevator channel.

Not a big problem, but after you test fly the plane it may take you a few trial and error linkage adjustments to get the elevator halves synchronized at neutral.

The reversing Y-harness will allow the trim tab to work both servos.