I agree about the difference between doubles and a threesome. Two gals that actually get into each other and you get to take your time and watch and then get to participate with them both is pretty damn hot and about as sensual as it gets.
As to picking the providers themselves, Rachel you are the epitome of a lovely provider and your counterpart could be whatever type of gal you like (depending on your own deep desires and what really just turns you on). As a hobbyist, I'd prefer the gals pick each other as they know what they like and it is potentially much more enjoyable for everyone.
The thought of watching the two gals make each other cum (orally, toys/strapon, etc) is pretty erotic and then going over and licking them both clean makes me lustful as hell. Toss in a little light bondage for one gal and and it becomes even more erotic as she gets "used".
And to have one gal straddle me CG style, and her kissing the other gal, sucking tits, etc while that gal face-sits you as you suck/lick her pussy is on my own bucket dream list.
Now, to try an clear my thoughts and hope my now wet hard-on vanishes...
(I'd settle for a daty session and a provider in stockings or yoga pants at the moment - lol)