I have a combo bass amp and a combo guitar amp. Unfortunately the guitar amp's tone cuts in and out, and sounds really bad, though the speaker is in good shape.

So the question is, will it damage my guitar amp speaker if I use it as a external speaker lined from my bass amp? Playing guitar of course, not bass
sɹǝʇndɯoɔ ɥʇıʍ poob ʇou ɯı uʍop ǝpısdn sıɥʇ sı ʎɥʍ pob ɥo
No, but why don’t you just play the guitar through the bass combo? A guitar won’t damage the woofer.