My Odd 1 off touring bike being built in Eugene in 90, and using the same dropout as Burly's tandems ,
I can say the vertical dropout is fine with a QR axle .. I used, and continue to, one of Bullseye's hubsets .
after being happy with a Phil Wood freewheel hub before that ..
But , the overwhelming market preference these days is Cassette freehubs .. these days tandem ones got threaded for the
now Discontinued , Arai drag Drum Brakes . if you've got those you may continue using them (if you can unscrew it.)
in 48 spoke and 40..