A tie away to either Portsmouth or Aldershot Town is the prize as both 'Dale and the Cobblers look to bounce back from weekend league defeats.
Northampton manager Chris Wilder felt his side had only themselves to blame in Friday night's reverse at Cambridge after the hosts equalised within a minute of Ivan Toney's goal before going on to take all three points.
In League One, meanwhile, Rochdale were edged out by a single goal at Port Vale.
"There was nothing wrong with the performance apart from the fact we lacked a cutting edge," said manager Keith Hill afterwards.
"The boys played some really good football again and it's a shame we have ended up empty-handed.
"We played as well as we did when we beat Preston and Yeovil recently, but we just didn't take our chances and that is obviously a vital part of the game."