If Marvin and Grayson really are the top 2 players in the country, then we are almost a lock to be a #1 seed in the NCAA tournament, according to historical precedence. However, it is far from a guarantee of postseason success.
Here's a complete (to the best of my knowledge) list of teammates who were both AP 1st team All-Americans in the same season:
2010: John Wall & DeMarcus Cousins (Sweet 16 - HAHA)
2006: JJ Redick & Shelden Williams (Elite 8)
2001: Shane Battier & Jason Williams (National Champions)
1998: Raef LaFrentz & Paul Pierce (Round of 32)
1998: Mike Bibby & Miles Simon (Elite 8)
1991: Stacey Augmon and Larry Johnson (Lost to Duke
)
1984: Michael Jordan and Sam Perkins (Sweet 16)
1976: Scott May & Kent Benson (Undefeated)
1974: Bill Walton & Jamaal Wilkes (Final Four)
Interestingly, only 2 such teams have won it all: Duke in 2001, and Indiana in 1976.