Quote ="Steve Fox"In the years after WW2 challenge cup first round ties were two legged as were premiership semi finals in the late 1970s / early 80s.'"
They started in 1941-42 when Broughton Rangers, Leigh, Liverpool Stanley, Salford, Swinton and Warrington all dropped out of playing. Thus due to a lack of teams in Lancashire - Wigan, St Helens and Oldham entered the Yorkshire Cup and it was deemed fairer for those sides to play the ties over two legs. The Challenge Cup followed suit later that year and so did the Championship Play Off final.
After the War had ended and full competition returned - the first rounds of the Challenge and County Cups remained over two legs up to and including 1953-1954.
Then in 1969-70 the Preliminary Round of the BBC2 Floodlit Trophy was held over two legs up to 1973-74.
I'm not sure the Premiership games were ever over two legs myself, but might be wrong?