not exactly what you want, but close, and extremely simple.
this will actually increase the outboard engine, and decrease the inboard engine. to do exactly as you want (only decrease inboard engine) will require throttle curves... if you want that, i can put that in an additional post.
this assumes you are starting with a blank model with no mixes.
go to your mixes page
add a mix to channel 1, use AIL for source, set weight to 100%
add a mix to channel 3, use THR for source, set weight to 100%
add a mix to channel 4, use THR for source, set weight to 100%
to channel 3 add an additional mix which uses AIL as its source with +30 for its weight
to channel 4 add an additional mix which uses AIL as its source with -30 for its weight
if the throttle drop/add doesn't give enough kick, or gives too much kick, adjust the weights on the additional mixes for channel 3 and 4
if you want to have a separate throttle steering from the rudder steering, use different source for additional mixes on channels 3 and 4. i will use RUD as my source on 3 and 4, and AIL for my source on steering, so that i get engine steering on the left stick, and rudder steering on the right stick.