I was involved with someone else for a while during separation. It can last a long time sometimes and I felt lonely and behind in life. At first, I thought I would just completely move on and forget about them. When it (quickly) became clear that I wasn’t just going to move on, I was honest with the third party. They didn’t have a problem with it, they said they were in a similar situation (though I didn’t use the twin flame terminology).
I think as long as you are being up front with how you are committed to the relationship, it is perfectly fine.
I’m with my twin these days and I’m actually still on speaking terms with this third party. They were a great person, just not my person. I don’t regret my time with them but I think I would if I hadn’t been honest with them.