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I'm not sure if limiting the number of options selected in a multi-select filter is possible. A possible work around to this would be to create 2 parameters that allow the user to select their two choices, one choice from each parameter list. You could then use a calculated field like the following as the filter:
if [Field]=[Parameter 1] or [Field]=[Parameter 2] then "Show" end
I would include "none" as an option for the second parameter in case a user only wanted to select a single option. This might not be the ideal solution, but it is one that I've used before when wanting to place a limit on the number of options which can be selected.
Hope this helps