Coax and twinax, etc., are pretty simple, it's just the nomenclature that might trip someone up. Coax has two conductions: the center and the shield. But they are usually described as a single conductor plus drain (shield). Don't be fooled -- the drain is a conductor! And it is insulated. The only difference is the conductor is spread out on the surface of a cylinder. This can be done by using a foil, or a foil plus solid wire, or various forms of braids. Drains are also rated by how complete of coverage they provide.
Twinax is two signal conductors in the center of the drain (shield) conductor. So, there are 3 conductors, but it's usually called two conductor plus drain, as you only get two signal conductors.
For stereo, you need two coax. A typical RCA cable is two coax conductors with the outer rubber melded together for convenience. It is not twinax. As others have mentioned, RCA is the name of the connector not the cable, but consumers usually don't think of it like that. They know "RCA" for analog stereo interconnects and "coax" for spidf digital interconnect.
Pretty much all low voltage, high impedance systems (like stereo line level) wants to use coax for the shielding. That's because a fairly low power noise coupling can show up on the output. Speakers not so much, as they are driven by current, not voltage, and coupled noise will generally be low power. Of course, really long speaker wires can pick up enough noise to be audible, thus step up/down transformers....