There is not one solution to this - which is the right one depends on many things... like maybe the need to "save" on mixes...

I use the T18SZ and always have at least following conditions: Off, Taxi and Flight. Starting engines is done while in taxi; from here I can select the Startup condition). The reason is I don't want a switch that would put one engine on idle while airborne - in flight mode that switch does nothing (or maybe something else).

A solution could be:

In Linkage menu -> Function have J3 control both your throttle channels. Assign trim to both throttle channels if you want to be able to adjust idle of each engine. Exit menu.

In Model menu -> Condition select create the conditions you want and assign a switch. I assume you have at least Taxi and Flight. Exit menu

Select Taxi condition and enter Model menu -> Dual Rate. Assign D/R for throttle1 to switch SC rear position. Do the same for throttle2 but with switch in the opposite position.Exit menu.

With Taxi selected and switch SC in rear position, enter Model menu -> AFR. Select Throttle1 and go to page 2. Set rates to 0 and offset to -100 (or -95 if you want high idle on the running engine while starting the other). The the same with SC in forward position for throttle2

That's it - you have a switch (SC) that separates the selected throttle from J3 while in the forward or rear position; in the center position J3 controls both throttles.
The throttle trim is respected no matter the position of SC (unless you opted to for -95 to raise the disconnected throttle)