Yes, you can!
I'm not sure about tube amp but you can do this with two SS amps connecting speaker between +outputs of both amps and shorting -outs (grounds) together. Since this is a little similar to bridging amps (but no phase reversal) amps have to be bridgeable (single ended outputs)and have similar power ratings. Both amps have to be on and the speaker will receive difference of both outputs. You can even play both at the same time. I've never tried this (strange application) but don't see any reason why it wouldn't work. Unpowered amp can get damaged but it's the same with two monoblocks bridged for more power. On the other hand it's always better to keep things standard and avoid experimentation.
I'm not sure about tube amp but you can do this with two SS amps connecting speaker between +outputs of both amps and shorting -outs (grounds) together. Since this is a little similar to bridging amps (but no phase reversal) amps have to be bridgeable (single ended outputs)and have similar power ratings. Both amps have to be on and the speaker will receive difference of both outputs. You can even play both at the same time. I've never tried this (strange application) but don't see any reason why it wouldn't work. Unpowered amp can get damaged but it's the same with two monoblocks bridged for more power. On the other hand it's always better to keep things standard and avoid experimentation.