Will Shimano Di2 synchro shift work with an XT (Mountain) rear derailleur with an Ultegra (Road) front derailleur on the same bike?
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.