Accepting both mono and stereo 1/4 jacks without a switch

I want to make an instrument amp with a 2 channel amp board that I have, that will drive two speakers.

I would like to have a single 1/4 jack input, that can accept either:

1) a mono 1/4 jack cable, in which case, the signal gets duplicated to both channels

2) a stereo 1/4 jack cable, in which case, each channel

Is there a way to wire a 1/4-stereo female connector that can do this ?

I know it can be done with 2 seperate 1/4 inputs (one stereo and one mono), and a switch, but it would be nice if it could be done without a switch, and just a 1/4 stereo jack input.