So I am a multiple repeat customer with two ladies who are really close.  One introduced me to the other.  Both see me at a rate that is not published and is significantly below market.  I think they have made a business decision to keep me as a long term repeat customer with these rates.  Let's call them L1 and L2.   The point here is that I know each of them really well.  

I would ask L1 what are her expectations first, and if they are reasonable "conference in" L2 to the conversation and get her thoughts on the subject. Who knows, you might be in for the weekend of your life for virtual peanuts.  

If this is something you want to do, do it. If not, don't do it.  

It is very unlikely that they'll do anything to hurt your long-time relationships. So, unless they are both planning to retire and want a free vacation on you, it sounds like a great time.

What you (the OP) DON'T want is to merely be the financier for these two ladies to have a nice weekend together.  As GaG says, if they are including you in on all activities, then it might be a once in a lifetime experience.  For me, it would come down to the numbers.  Since I can't fuck all day long (I'm speaking only for myself, and not GaG), if they want to hit it three or four times a day and I was only a spectator some of the time, it would still be pretty hot.  I would make sure you're all staying in the same room, with an oversized king bed (you know, the kind that sleeps three and fucks six).  Accordingly, the financial expectations of the ladies would have to work out to paying for a couple of hours each per day, and the rest of the time is OTC.  If they're both expecting a 12-hour or overnight rate everyday, and/or want some alone time with just the two of them, I would pass, but my pockets might not be as deep as yours.  My hobby budget averages  about $8000 a month, but I would be loathe to blow it all on a three-day double, no matter who the girls were.

You've already seen them both. I assume you're footing the bill for the hotels, transportation, and meals. If not, you may want to work those details out ahead of time, to include the donation(s).

... reminding me of a truth I have heard before, and experienced all too painfully. Paraphrasing: It's not the things I did, but the things I missed out or chickened out on that I regret.

Two women you really like would like to go with you some place and spend several days wearing you and each other out.  Are you waiting for a better offer?  Just out of curiosity what would that look like..... well I GUESS they could pay your way....

Two of my ATF suggested we think about a three way on different occasions and I was the one to push to explore it.  Finally agreed on time and day and went at it.  Was great but realized I am not a threesome type of guy.

Sounds fun, but I would make sure every thing is spelled out.  Especially as far as donations.

Your handle is hilarious I love it

For your normal session length that you enjoy with each lady separately. That seems like a practical and cost effective way to get the same thrill. As mentioned previously, you can only expend so many bullets in a day anyway! If a few of those sessions live up to your fantasy, then consider a multi-day trip.  Plenty of things can go wrong on a multi-day excursion including ballooning costs, and perhaps the three of you do not get along as well as hoped. I see some great ladies, but sometimes things are not as great after I finish early! For me the great part of the threesome is four hands and two mouths exploring my body while I'm doing the same with all of my appendages, not the scintillating conversation before or after the act!

No, I fully expect to pay something.  What I mean is that I believe that they will attempt to make this reasonably affordable, since based upon all of our individual sessions, that we can expect a really good time.  I get that this is still pay for play.  Since I posted this, I have already found out that my fees would be well below the published rack rate.  So in short, I don't think they see me as a giant ATM, where they are trying to extract every last dollar out of me.  

IOW, lets say he currently sees each lady for sessions of a few hundred dollars per session, a "money grab" would be one or both of them "suggesting" a trip where he'd be spending $10,000 or even more.  

You nailed it. L1 and I have a great relationship, bordering on friendship. I know I am still a client, but that doesn't mean that we can't enjoy OTC fun.  Almost a year ago, I wanted to have her with me to celebrate a big business win. I offered to pay extra for her time at a dinner.  She told me to just pay her normal BCD rate,  and that she didn't want anything extra for dinner and drinks.   I could have taken the cynical approach and concluded she had some kind of agenda. A year later, I am still a client, and have concluded that she had no agenda.  We simply have a great time together.  She confides in me about stuff going on her life, that she is able to share with few others.  L2 is one of those people.

Some people seem to think there is no middle ground between being simply a paycheck and being in love. There are a LOT of gray areas in between. I have had more than a couple of favorites over the years that I ALWAYS paid for BCD time, there was never any illusion/delusion that we were BF/GF, but we'd spend countless hours either before or after our paid BCD time together being friends. Dinner, drinks, movies or whatever, our dates always lasted several hours, but only the time between the sheets was paid for.