Harris Cyclery does sell a made-up 13-34. I think what I'll try is making up a 13-34 from a combination of my 13-30 and 11-34, but it may be that too many of the cogs in the 11-34 are pinned. If we were a few years younger we would never need the 34, but such is the nature of life. We've only been riding the tandem for 1 year so I'm holding out hope that we'll get strong enough to ditch the 34.
We do spin out a 60-13 at about 32 mph, but it's unusual for us to feel compelled to pedal above this speed, maybe on a long 3% downhill. Much above that grade our just-right 350 lb. team weight does all the heavy lifting.
I did use my 36t chainring more on today's ride in the rollers. This chainring puts me in the tighter rear spacing. The daVinci has a longer chainline than a standard tandem so the cross chaining in this configuration is not so objectionable.