FWIW, our Peugeot's original high gear was 52/14, and it still is. I know some couples really fly along on their tandems. We have seen the low 30s mph occasionally downhill but never on a road which appears flat enough to pedal instead of freewheel down. Though I have shifted into the 52/14 combination once or twice for the heck of it I have never felt a real need for it. On the right kind of road we will pedal in the low to mid 20's mph, but even that is usually with the 52/17 (or whatever the next cog is). I run "compact doubles" on my solo bikes too for the same reasons, a high of 48/14 which I almost never use and never actually need. (The Bianchi and the Masi may make me have to alter that statement a little.) I ain't racin' anyone, and around here if I/we ever get going so fast I/we spin out the 52/17 then I will likely to have to downshift again in less than 12 parsecs anyway.

That's what I tell myself, and it works well until I make one unthinking RD downshift shift when I'm grinding uphill on a road with heavy traffic. A few moments later I discover what gear I'm in. That's when I'm really glad I set up the chain to accommodate the big-big combination. Otherwise it might have broken something. I don't worry about cross-chaining the other way though because I almost never use the little cog anyway.

One final thought. About balance, I have been surprised at how easy it is to balance, maybe even more stable than a solo bike. Riding slow is not a problem.