Another set of real twins, Camilla and Carey More appeared on Days in 1986. Camilla More played Gillian Forrester, Shane's ISA partner for a hot minute. She was in love with him and killed his ex-wife, Emma Donovan Marshall, then framed Kimberly for the murder. Gillian had her identical twin, Grace Forrester, (played by Carey Moore) come to Salem as her alibi for the murder. Once the truth was found out, Gillian went to prison for Emma's murder. Carey More (Grace) left the show, and Camilla took over the role of Grace. Later, Grace was killed by Harper Deveraux as the Riverfront Knifer.
I think the only other adult twins on a soap were on Passions, but they didn't appear together. Bruce Hall played a character named Reese. When he left Passions for another soap, Passions hired his identical twin brother, Seth Hall, to take over the role of Reese.
I'm excluding all the twins/triplets/quads who've played soap babies/children throughout the years, since they shared the roles of single characters, except in the few instances where twin children played twins on the show, like when real fraternal twins Tiffany and Edwin Palma played Sami and Eric as toddlers, and the twin babies who played Victor II and Joseph Kiriakis. Or the various sets of identical twins who played Steffy and Phoebe on B&B until the show decided to make them fraternal twins as young adults and hired two non-related actresses to play them. Or the twin toddlers who played Bridget and Michelle McKinnon on ATWT for a handful of episodes (the identical twin daughters of Jake and Vicky).
In the 80's, a couple on ATWT had a set of quadruplets. Not sure if they were played by real quads, two sets of twins, or just a bunch of unrelated babies.