Used to be called 'dual tows'. Was done regularly at RAFGSA clubs in the 1960's behind Auster & Chipmunk tugs (not exactly overpowered) mostly for aerotow retrieving in the days when there were rather more straight line distance flights, or taking two gliders 50-100km into wind to do 300 km downwind before hitting the coastline. I haven't done one since 1972, but it was very trouble free then.