Did Mentzer and Gygax play together?
Did Gary Gygax and Frank Mentzer play together in one more sessions of D&D (or some other roleplaying game)?
They worked at TSR and co-authored D&D products. One would expect one of them to have run a game for the other, or them to have played in a shared game, but expectation is not the same as knowledge.
The context for the question is frivolous: this is relevant for determining Gygax numbers (in analogue to Erdős and Bacon numbers).