Chief Inspector Rob Leet, 44, and Sergeant Sarah Porter, 38, worked for Sussex Police and have been accused of gross misconduct.
The Independent Office for Police Conduct (IOPC) said: “The officer is said to have met with a woman he knew to be a former victim of domestic violence in late 2014 and early 2016 for sex while he was on duty.
“He is also alleged to have met with a police sergeant on at least one occasion the following year to engage in sexual activity while both officers were on duty.”
The watchdog said the two officers allegedly travelled to meet up during work hours and repeatedly used Sussex Police communications systems to exchange messages “where there was no work purpose”.
Sgt Porter is also accused of inappropriately contacting a witness to the IOPC investigation, and of failing to be contactable after a fatal car crash – despite being on duty.
Both officers were referred to the IOPC by Sussex Police in March 2017 and face a gross misconduct hearing after officials found the allegations would violate standards of authority, respect, and duties and responsibilities.
CI Leet was married with four children at the time and Sgt Porter had a husband and daughter.
IOPC regional director Sarah Green said: “Our opinion is that, should the allegations be proven, they would amount to gross misconduct and Sussex Police has agreed.
“It will now be down to a panel, chaired by an independent legally qualified chair, to hear all the evidence and decide whether the allegations are proven.”
A spokesperson for the force said: “Sussex Police can confirm that two officers who have been the subject of an independent investigation by the IOPC will now face a gross misconduct hearing at a date to be arranged. Details of the hearing will appear [online].”