Lorraine Chase (right) and Caroline Strong (below) - who played the Dales' Steph Forsythe and Melanie Doland - appear in an episode at the beginning of September and take on the roles of a prostitute and an aggrieved daughter.
As Melanie Webber (Strong) searches through her late father Stanley's belongings, she's surprised to find the name 'Carol' repeatedly scrawled through his diary.
Her search for 'Carol' leads her to the Campus and to Cherry Malone (Sophie Abelson), who she mentored at college.
On returning to Stanley's house, they're perturbed when they hear someone let themselves into the house. They're further disturbed when they walk in on Carol Dunderdale (Chase) watering the plants.
Melanie's outraged to hear about her father's indiscretion, but Carol admits that her relationship with Stanley quickly turned into an innocent friendship and her inheritance has been put to positive use.
Chase and Strong's episode, titled 'His Coy Mistress', airs on Thursday, September 10 at 1.45pm on BBC One.
Chase quit Emmerdale in 2006 after four years, while Strong was written out of the show in July last year at the close of the soap's baby swap plot.
Earlier this week, it was announced that Some Mothers Do 'Ave 'Em star Michele Dotrice is to make a guest appearance during the same week.