If you see just one, and pal sees other, seems to me, each should pay their girl's rate.  If you switch girls, then it could get fuzzy.  Don't think this is a common happening, so play it by ear.  Don't know what else to tell you.  From what I've read, strippers are not usually providing outside the club, or inside it either.  If it worked out for you guys, more power to ya.

Swim

Let the girl arrange her friend based on your friend's preferences... (obviously both people you and your friend will be screened), typically the girl I bring is on the same donation bracket so rates are not an issue as we both know each other well, have two separate envelopes ready for the total time you are looking to spend as a group (typically two hours for each girl, one hour with each person).. These arrangements are not meant to be an orgy FYI. rather...the opportunity to maybe see both girls in action but highly doubt it you would like to see your friend in action, reason why I encourage gentlemen to get separate hotel rooms so each party has their privacy.
After the negotiated time with each gentleman is over, we will freshen up, have a drink, talk and part ways again.. so it can be arranged in a civilized way, but I do not see the price benefit because in reality this is considered doubles, and obviously dinner will be also have to be considered into the donation.. Once again, the lady you plan to see will give you more specifics.