Had a fantastic time twith two providers, but only one wanted me to write a review (the other did not). Do I review just the one as if she were a sole provider at the time?

And one of them is not writing a double review as if it was a single. I dont think that would be right.
Option one is what Julia said. See the one lady (the one that wants the review) again as a single and review that date.
Option two is to write up the double review as a double and leaving one girl anonymous. The profile for doubles is only going to be for one of the ladies anyway. So you write it up and submit under her name. (the one that wants the review, of course) Then, when you write up the body of the review, you only name the one. Let's use Julia as an example since she posted on this thread  You would write it up as "I had a doubles session with Julia and a UTR provider" or "Julia and a provider that wishes to remain anonymous." In describing details you say something like, "while Julia was ... The other lady ..."

I have read a review like that before so I don't think it is against TER policy, and you would be honoring both ladies' wishes.
My.02

It's an example of a review with a named provider and her friend

write a review and place both names in the nane box, like 'Suzy/Janie'.  this will create a profile for these two girls as a duo.  It will create a profile for them as a double.  It will not appear on their individual profiles.  If you want to do a review of either one by herself, it may not be approved, it you do a review of them together.  With the changes TER has instituted, it's hard to say what will happen.

Swim

I knew I read reviews like that, and thought about posting a link to my post above, as an example, but I was too lazy to search for one. Hopefully my post and your link will help the OP.

Then again maybe we took the OP's question differently. My take was that he had a duo with one "public" provider, and one UTR provider. If that is the case, he should not name the UTR lady.

shouldn't be written.

The only exception is if the ladies didn't meet expectations.

If what you say is the case, and it likely may be, you are correct.  If one lady does not want her name made public, then he should not name her in the review.  He does the review under the name of the other lady and can say that they were joined by another w/o mentioning her name.

Swim

We just each had a different take on the OP. I couldn't imagine you would suggest naming a UTR lady.