You write
"Suppose, plausibly, that what it is for two experiences to be subjectively indistinguishable is that there exists some one-to-one mapping among the instants that make up those experiences so that you can’t tell apart any instants mapped to one another."
You note that there is a one-to-one mapping between a continuous one-second-pain and continuous two-second-pain, while the two-second-pain seems obviously worse.
Consider the parody principle "what it is for two ranges of numbers to be mathematically indistinguishable is that there exists some one-to-one mapping among the numbers that make up the two ranges". This principle is of course false (0 to 1 vs 0 to 2).
Many people might consider the parody principle plausible. Do you have a reason in mind for thinking that the mistaken intuition supporting the parody principle isn't also the primary intuition supporting your principle?
You write "Suppose, plausibly, that what it is for two experiences to be subjectively indistinguishable is that there exists some one-to-one mapping among the instants that make up those experiences so that you can’t tell apart any instants mapped to one another." You note that there is a one-to-one mapping between a continuous one-second-pain and continuous two-second-pain, while the two-second-pain seems obviously worse.
Consider the parody principle "what it is for two ranges of numbers to be mathematically indistinguishable is that there exists some one-to-one mapping among the numbers that make up the two ranges". This principle is of course false (0 to 1 vs 0 to 2).
Many people might consider the parody principle plausible. Do you have a reason in mind for thinking that the mistaken intuition supporting the parody principle isn't also the primary intuition supporting your principle?