There's a greater chance Biden shows up than Trump. I can totally see Trump making up some requirement that no one with a brain would comply with, then refusing to show up when Biden doesn't comply.
Trump has already demanded Biden take a drug test because he doesn't want Biden "on whatever he was for the SOTU". So that's a potential backout reason right hter.
Calling it now: if (a) Biden takes a drug test (and of course if he does, Trump should too), (b) the test comes back clean, and (c) Biden does well anyway, then the Usual Suspects will claim that Biden took a top-secret
undetectable drug.