It is useful to go in with an outline plan but do not try to follow it rigidly.
Think about what you want - things I have enjoyed include.
Lesbian show
Dual blow job
Both girls kneeling in doggy position and taking it in turns to fuck them
Similar to above but smaller girl lies on top of larger with their pussies close together and swap between them
Toy show - I once took a cucumber to a duo (agreed before booking) which the girls both inserted and had fun with. When I fucked one of them straight after her pussy was chilly which was a novel sensation (vampire cock according to the girls)
One key thing to know is whether the girls will want to change condoms when you fuck them. I have never had this happen but apparently it is recommended for the girl's health (I take the view that they are professionals and if happy to not swap that is ok with me).
Another thing is to pace yourself - on my first threesome I came before I wanted to and, although I managed another pop, I would have preferred to have built up through the meet to one big climax.
Unfortunately the threesomes I had were with girls who have long retired or moved on.
Finally try to relax and have fun - I look forward to reading the report