You'll have to do custom cables for throttle anyways, but that would be easy enough. I'd stick to a mechanical shift linkage.
If you rig up a shifter bar like shown, and make sure the throw ends up being the same (several ways to design it in), it will shift just the same. I'd recommend good sealed bearings at the shifter and some VERY hard mounts, since the main problem will actually be from the engine torquing. In other words, the stabilizer bar will be the harder part to replicate. One way to combat that is to make the rear shift throw shorter (engages quicker), and add in a rubber or spring so it can compress so slightly after engaging, to account for any engine movement.
I wouldn't worry about "desyncing". You won't get a perfect 1:1 based on weight trasfer alone [deforming tires]. If one engine desyncs, it will just act like it's an AWD system being fed more or less torque to one end. So one engine will be slightly more loaded than the other. If one is 50rpm behind at redline, it will just be shifted that much before it's own redline.
Forgot to say, I'd actually run a hydro clutch system. Less pedal effort, easier to design, and - you can use something similar to a line-lock to keep the rear clutch out if you want to run 2wd with one engine off. The t-out bearing wouldn't be moving, since the PP isn't with engine off, so no added wear.