How about, "You should X, and you should accept a standard that would recommend it?" Thereby appealing to a third (shared) standard, possibly one having to do with rationality of moral beliefs. Applying an analogous moral version of Aumann's Agreeement Theorem could lead us to a theory which suggests that you can never say this quoted sentence unless you're willing to believe that you should accept the standard you recommend.

I do hope to avoid discussion about the common usage of "should" in favor of a theory that would allow us (if no one else) to use it consistently to refer to some shared standard, and I believe this can be done without paradox. So long as a community shares a sufficiently basic belief, it will be possible to extract shared consequences of that belief. In the same sense that a group of rationalists cannot convince non-Baysians that they should apply Aumann's Agreement Theorem, we cannot convince an analogous group that our word "should" refers to our internally normative values. In neither case should we worry.