Let the starting point be O

After 6 seconds:

Distance traveled by the ant travelling north = $2 * 6 = 12$ inches
(Let A be the end point of this ant)

Distance traveled by the ant travelling north = $12 + 12 * 1/3 = 16$ inches
(Let B be the end point of this ant)

Observe that OAB forms a right-triangle with $angle AOB = 90^o$

Thus, distance between the 2 ants (from Pythagoras' theorem) = $\sqrt{12^2 + 16^2} = 20$ inches

Thus, distance in feet = $20/12 = 1.67$ feet