Here's a strange 1. Twin fountain stock drive trim. When the drives are place in forward (don't know about reverse) and the throttles are advanced slightly past idle, the drives will not trim. Both trim normally both individually and on the throttle when in neutral or at idle in forward. Any guess?

Sounds like a broken wire in the underneath side of the throttles. Easy to trouble shoot with a ohm meter at the underneath side. Just check before the diodes.

The shifter position doesn't matter. It's the throttle lever with the switch that is what is making/breaking the wire.

Great, I'll give it a shot. Thanks.