A provider posed this question to me a while back and I thought it was a good one. To me, while I have specific things in mind, the general, short answer is sexual interaction between the ladies. While I certain enjoy the attention of say, a double BBBJ or four hands all over Spanky Jr., I don't see myself ever knowingly signing up for a double where the ladies don't even touch each other. Ideally I'd like for the ladies to kiss, DATY, and partake of some threesome-specific positions.
I'm not naive enough to think that all ladies are really going to be into it, but good acting is sufficient. I have been fortunate enough (once) to be with two ladies who were avowed bisexuals and it was amazing.
So that's what I think makes a good double. It would be interesting to hear your thoughts, both clients and providers.