Fundamentals check on rating switches (and switch-like elements):

(in fundamentals land, assume ideal elements unless necessary to do otherwise)

(There is a picture attached)

Imagine two simple switches [S1 and S2] in series, with a resistor [R] between them. Imagine that a potential is developed across the two opposite unconnected ends of the switch [V1-V2].

When both switches are closed, the voltage across each switch is 0V, the voltage across the resistor is [V1-V2], and the current through each switch (and the resistor) is [V1-V2] / R. The switch has to carry that current.

When S1 is opened and S2 closed, current drops to zero. The voltage across S1 is [V1-V2]. The voltage across R + S2 has to be zero, as no current is flowing through that branch. The switch S1 has to withstand [V1-V2].

When S2 is opened and S1 closed, the situation above is repeated for S2: S2 has to withstand [V2-V1].

When S1 and S2 are both opened, no current flows through resistor R; therefore the voltage drop across R is 0. The withstand voltage of S1 + S2 has to be [V1-V2]. If the switches are in all ways equal, the withstand voltage can be [V1-V2] / 2 per switch.

... in posting, I think I Rubber Duck'd my original question (what are the minimum voltage ratings for two open switches). Please let me know if you disagree with any of the above.