The pair will join other senior executives to form a new creative leadership team, collectively responsible for the 110-strong department.
Bitsack has worked at JWT for the past three years, handling brands such as JetBlue, Stride gum and Domino's. He previously freelanced at a number of agencies, including Bartle Bogle Hegarty and Wieden & Kennedy.
Wiliamson rejoined JWT last December after helping the agency win the lead creative assignment on Royal Caribbean's global account.
Bitsack and Williamson will report to Joe Cabaco, Euro RSCG's North American chief creative officer.