Using solid copper Door Bell wire as speaker wire.

Doorbell wire is 20awg, but thermostat wire is 18awg. Doorbell wire comes in pairs and thermostat wire comes in a variety of configurations including a pair, many odd numbered parings but also an 8 wire version.
If you took the 8-wire version, used every other wire to each of the two poles (4 wires per pole, pos and neg) then you would have an aggregate 12awg cable. The skin effect brigade will like the doorbell wire better. With that, I would use four of the twisted pairs, themselves twisted in a star-quad configuration so also 4 wires per pole, for an aggregate 14awg. Arguably, this would be the better cable of the two.
With either, use two separate runs for bi-wire. If you can find the wire with polyethylene insulation, that would be much better, IMO, than PE and even better than teflon. You could do much worse.

Audiolabyrinth, just because some manufacturer pastes their logo on it and puts it in a fancy bag/box/aluminum case, doesn't make it particularly better. Cables consist of copper (or silver) wire, dielectric material, geometry, damping materials, shielding (or not) and connectors. I understand there is research behind most of the top cables, but there is also a lot of marketing spin and fairy dust. You don't have to pay a lot of money to get good sound. I have heard, many upper range cables from manufacturers like Purist, Cardas, Audioquest, AZ, and others and, mostly, I prefer cables I have made myself. If the OP’s doorbell or thermostat wire is CDA 101 OHFC copper, and the dielectric is PE, this is about as good as is being used by many of the top cable makers. That it is solid core gives it an additional boost in my opinion. You shouldn't make the guy feel "unworthy" just because he isn't spending a bundle on cables. We will never agree because IMO the impact of a cable will never equal the impact of a component on how a system sounds.

Smaller gauge wires work fine for speaker cable, you just need to use a lot of them.

HT used 19, 24 awg OCC copper wires insulated with air formed polyethylene, per leg, in their Pro-11 plus cable. In their new line of cables, including the new version of Pro-11, they have taken to using multiple, individually insulated wires in a range of sizes from 20 to 24 awg. Not all that different from cables that could be made using multiple doorbell wires.