duos (sticking my big nose in again)difficult to decide in my humble opinion,some seem to just work as duo and dont push their luck,some take the mickey and have twice as many songs often long winded.If they are good i as a listener and occasional player would mind but if they are not so hot then its that difficult bit.If I were a host id say as duo 3 songs or 12 minutes on your cambridge principle,then its fair.My pal was at one singsong and a group of morris men after leaping about hitting each other outside came in for singaround but had one song each-whole gaggle of em!Apparantly after 3 unaccompanied 20 verse renditions a lot of the pub needed a breath of fresh air outside or toilet visitations but they carried on regardless-now thats a bit out of order,come on morris men,defend your case!!!grumpy