If you think about these two options and the one RB describes as a logical truth-table, you will see that no matter how stupidly it is wired, they are the only possibilities for S1 gang 2 if it actually does something. The specially-controlled light can either be forced off (RB's scheme); on (my first scheme); or to the opposite state to the 2-way (my second scheme*). The only remaining possibility is to force it to the same state in which case the switch does nothing at all.
* strictly speaking a variant of this scheme, using an intermediate as S1 gang 1, with the 2-way light teed into one outgoing strapper so that it functions as 2-way with S2, but both strappers continuing to a 2-way switch as gang 2 with the light fed from the common. Then, both gangs of S1 control the special light, gang 1 as 2-way with S2, gang 2 then selects 'same or different' to the 2-way light. Yes you would need a grid switch, no it's not very sensible!
* strictly speaking a variant of this scheme, using an intermediate as S1 gang 1, with the 2-way light teed into one outgoing strapper so that it functions as 2-way with S2, but both strappers continuing to a 2-way switch as gang 2 with the light fed from the common. Then, both gangs of S1 control the special light, gang 1 as 2-way with S2, gang 2 then selects 'same or different' to the 2-way light. Yes you would need a grid switch, no it's not very sensible!
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