A single tube can have multiple sections (that’s what they actually called). Something like the 6SN7, which is what you’ll see in a Lyr 3, is what’s called a dual-triode - because it has two independent triode’s (6J5 in this case) inside that can be accessed separately.
With dual tubes that have the same type of triode (or pentode) for both sections, you can use one section for each channel. And when doing this, and actually buying tubes, you want to make sure you get “matched sections” to they have gain that matches as closely as possible.
Some tubes have multiple sections that are different, such as the 13EM7, and those are not suitable for driving two channels from a single tube.
You’ll see dual-tubes mostly in small signal tubes, but they do exist as power tubes as well, such as the 6080/6AS7 dual-power-triode used in the Bottlehead Crack (which also uses a 12AU7 dual-triode).