Will Shimano Di2 synchro shift work with an XT (Mountain) rear derailleur with an Ultegra (Road) front derailleur on the same bike? Basically, can I mix a mountain Di2 rear derailleur (specifically, the RD-M8050-GS) and with a road Di2 front derailleur (specifically, the FD-R8050)? Will firmware updates and syncro shift still both work properly?

Full context: I'm trying to build a touring tandem with a very wide gearing ratio... hoping for an 11-42 tooth cassette and a 16 tooth differential in the front.

I'd use full syncro-shift to ensure I can't cross chain (at least after it finished shifting) to ensure the rear derailleur has sufficient chain wrap capacity.

The other option is to use either (1) all mountain derailleurs with a larger big chainring at a 16-tooth differential, but I understand the front XT derailleur is only supposed to be used on a 10-tooth chainring differential, or (2) all road derailleurs with an 11-42 mountain cassette, but the road rear derailleur is only spec'd up to a 34-tooth sprocket, and I'm not sure if a road derailleur with a Wolf Tooth RoadLink DM (to allow bigger cogs) is as good as a native mountain derailleur on an 11-42 cassette.
No, Shimano's software intentionally disallows you from setting your bike up with a mix of mountain and road derailers. Boycott-worthy, I say.

It's very common for touring tandemists to wind up feeling best served by doing something or other with their front chainring setup that is just never going to work well with front indexing or di2. If you look at serious, cost no object touring tandems that get ridden, you'll see a lot of downtube and bar end shifters.