I finally have a (good…) working solution for my 1972 ES-355TDSV! A TRS cable into a 1/4” Y splitter, used in reverse (to combine 2 signals rather than split 1 signal). It works quite well!
Drawings by graphic artist and illustrator DZO Oliver
"Attracted by all forms of artistic expression, it is first as a graphic designer I learns the techniques of image creation with a predilection for drawing and generally for anything that does not depend digital Design. The discovery of interactive pen displays however, offers me new horizons and now I do not impose any constraint in terms of pictures conception. Mixing styles and techniques is my way." [text from Behance]