Actually, the nominal impedance running a 6 ohm and an 8 ohm cabinet in parallel is about 3.4 ohms, rather closer to 4 ohms than 2. Additionally, many people knowledgeable about tube amps say that running a tube amp at a higher impedance than the load is better then running it at less. That is, it's better to run a 4 ohm load at the 8 ohm setting than vice versa, which is decidedly not the case with transistor amps. I don't claim to know the answer, myself, but there seems to be a broad consensus that up to 100% mismatch is usually okay on a tube amp.
Personally, I'd run it at 4 ohms rather than 2 in this situation. And yes, NsubUlysses, old Model T amps did go that low. My 1974 T has 2, 4, 8, and 16 ohm settings on the impedance selector.