We have been running a 1X on our tandem from the start. Our working cadence is the key to this. We have a 30 tooth chainring in front and a 11-50 9 speed cluster in back. Our wheels are 700C and currently we are running a 700X32C Conti GP5000. I originally had a 9-50 SRAM11 speed in back, but I switched to the Box 9 speed because I felt that the chain line angle with the 11 speed was the source of poor shifting. The Box 9 speed does indeed shift much better
With the 11 tooth sprocket we have a top speed under pedal power of right around 30 MPH. We can run a fairly high cadence, With the 50 tooth sprocket we have a 3 MPH crawl speed that has taken us up some fairly steep ( more than 10%hills. With a long steep downhill and gravity to pull us we have seen a top speed of between 45 and 50 MPH. We consider this acceptable for a touring tandem.
We are able to bridge the gaps in the gearing as our working cadence is from 75 - 105. We are both in our mid to late 60's and while we are in pretty good shape for people in the 65-70 year range we are not going to win any races. Our longest days on the bike have been in the 75 mile range and Mrs. Dan was ready to kill me at the end of those days. We have an agreement that no future tour will include such a day without approval of the Stoker.