Sneakily trying to steal property with no force whatsoever in an area separate from where the clerk even was is not a robbery if robbery is the felony you were thinking about.
Both stealing and conspiracy are felonies in Missouri, however, there is a monetary requirement of $500 before stealing becomes a class C felony, and if the "bank bag" in question had at least $500.00, then the two men could be charged with felony stealing and conspiracy.