The last time - not that long ago! - that a thread on a topic like this was current, I offered the advice one of our colleagues on the Northwest Boards gave me when I was trying to set up a more satisfying MFF than the one I'd had very early in my punting career; possibly before this Board existed.
The advice was simple: see both of the SPs individually before you book them as a Duo. That way, you'll know if you gel with them both. Also, you'll discover which of the two is the more dominant and you can then discuss with her how you want the hour/90 minutes to go. You're paying; so you get to call the shots!
I agree. Adds another dimension if you have already met them both. They both seem to put in the extra effort, up their game, outdo the other to be the better “regular”. You will be “processed” to completion if you are meeting for the first time.
If they work together in the same building, ie parlour or tour together then 30minutes is easy to arrange and fine for one pop, otherwise generally an hour minimum since it ain’t really worth their while, travelling, interrupting their usual routine.
I have done “one off” longer 2 hours duos only because they were advertised at discounted prices.
Don’t pay their full hourly rates since you are paying double for any downtime, including your showers, drink, phone, piss breaks. Better value to book an hour or two with your favourite regular and get the other to join for just half an hour.