Problem With Audio Cables

[anaxcrosswords]

I thought it would be simple.
3.5mm audio lead from speaker out to guitar amp (via double adapter, standard jack).
Jack cable from bass to guitar amp (via same double adapter).
Except it doesn't work. I can hear the music but not my bass. Tried swapping the two inputs, but same result - no bass. Removed the adapter and just plugged the bass into the amp, and it's fine. So everything's working, but the amp won't accept two sources into the same jack input.
Anyone got any suggestions?

[anaxcrosswords]

Got one, Fender! Well, not me personally - the band has one which we use for rehearsals.
Good idea, and as it turns out we have a spare. One channel failed so I got us a replacement. Assuming the old one hasn't been thrown out, I can nick that to use here at home.