If you want ladies in their 20s size 6 - 8, agency is probably going to be your best option as Nat and her duo partners are older and more towards size 12. There are some other AW duo options (Harley Lust and Carmen, Victoria and Kate) but they are also outside of your age/size range. McKenzies provide good information on what girls look like and what services they provide, but their duos are expensive and it is always a bit of a risk because you don't know how well girls will work together. IK list the girls who like working together, so experience wise this will work, but also expensive and somewhat vanilla. If you have time to organise, you're best bet is probably going to be methodically working your way through the adultwork profiles of girls in your age and size range to see if any of them say they can arrange duos and then messaging them to see what they can do. If money is less of a concern, I'd probably go agency.