It all depends on what the guy is interested in. Do they want all the attention on them? Do they like watching girl on girl? I think it's always best to let activities happen organically instead of having a checklist, but knowing what you prefer helps the ladies out.
In my opinion, I think it's better when:
1. You've seen both ladies previously and you already have an established friendship
2. The ladies are already friends in that special kinda way ;-)
3. Their styles compliment each other
4. They enjoy the experience just as much as the gent
And, if there's undeniable chemistry between, that's when magic happens
All that is why I enjoy playing with Kendall4U and Jessica UTR.