Anybody have a view on what providers charge to see a couple in relation to their regular rate?

Usually knock off 20 to 30% of the doubled price.
They make more and work less.

Couple rates are usually doubled.  2 people twice the work, and it is like, attending the movies your SO is never free!

That has not been my experience and I've done my share of doubles as evidenced by my reviews.
It's still one hour and instead of getting the single rate of 500 they get 800 for the same hour.
Now that's a bargain. It's also good marketing to entice the 800/hour donation.
Sort of like giving a discount for the second hour etc., etc. Maybe it's not like a movie but more like an airline where they discount the seat for a traveling partner.

It would be the same as a double sweetheart

as evidenced by the huge disparity in rates in general. I think the prevailing thought is, what works for some doesn`t apply to all.
Some will be sticklers for doubling the rate for the added participant, and some will not. I imagine there are a few, if not more factors a woman might consider.

Rate of client opportunities
Comfort level
Current financial position
Amount offered

I have to believe that a very busy lady with many options can hold out for a higher rate as opposed to a lady in a less secure position who might see the advantage of killing the same hour for a lower couples rate.

You are right. There are no absolutes. That being said
if someone's rate is 500/hour, getting 800 for that same
hour is always a home run and as I said before good
business.

Thanks for all the input.  Based on a very informal reach out to 4 providers in LA and NY who actually advertise their availability for couples, not one "doubled", and they ranged from minimal to 50pct premium.  But at the end of the day I guess it will always be up to the ladies, as it should be.  Thanks