The 1x system is pretty much intended for mountain biking, and cyclocross, where you don't need a big top end.

For a road tandem, its going to give you much bigger jumps than you currently have, and both a higher low end than you currently have, and a lower high end.

With a 42 front chainring, your top end will be down to 30 mph from 35 mph at 90 rpm on your current setup. Your low end will be up to 7mph at 90 rpm, from a little over 5mph now.

At least as significantly you'll be looking at double digit gear step percentages on every shift, in contrast to the single digit steps you now have for most of your shifts. Basically, most of the gear jumps will be like shifting two gears compared to your current setup.

I think you'll be much happier with Ultegra Di2 for not that much more money.