Update: the 6,3 mm mono female > 6,3 mm stereo male jack works like a charm! The biggest difference is that you have to use batteries. My Series I has the the pickup selector permanently on both (it's removed from the lower horn and hidden in the body cavity) so for pup-selection I'm already used to use the volume controls. I've found that if I want to use one pickup only , I have to add a tiny bit of volume for the other. I'm not technical at all, but there seems to be some kind of threshold. All in all a very easy and great solution!
Wilfred