The two stars are set to appear on the same episode, which begins production this week, according to The Hollywood Reporter.
The details of their characters are being kept under wraps.
The duo joins the previously announced guest stars including Tony winner Ben Platt, as well as returning faces Harry Connick Jr, Minnie Driver and Bobby Cannavale.
The show is set to return 11 years after it went off air, with all four series stars - Eric McCormack, Debra Messing, Megan Mullally and Sean Hayes, reprising their roles.
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