Doesn't answer your question directly, but for us among friends and the more plentiful acquaintances (which may be a couple we met an hour ago), we have used the slow, incremental approach, letting the women control it. Sex talk, clothing off, touching, my wife would provide an opening for the other woman to respond to and see what happens. In most cases the guy would be more in the role of just approving or holding back, not exploring the limits.
For example, the girls talk back and forth until they agree to go topless. Daniela gets the approval from his wife to put her tits within reach of her husband. The three of them can play feelies until the other couple both make it clear that I can touch his wife. Things can move quickly, slowly, or not at all. The goal is that wherever it ends up, no one feels pushed or has regrets.