Don't get me wrong. Like Papa Sweet says, this is a dream come true for most of us.
But watch out for those pesky little details. In the few times I've arranged a 3some in the past, I was careful to manage expectations, especially around allowance. If you are already giving the same allowance to each, this won't be a problem, other than your obligation to pay 2x for one (albeit one super awesome) BCD. If they are getting different amounts, well, you know they already compared notes on you including the financial stuff. I won't suggest you ask them upfront as that will almost guarantee you will need to increase one to get equity with the other. But be prepared just before any panties hit the floor, or just after, to pay the higher allowance to each one.
Next. what happens after? If the session is a total success, can you, should you, will you plan more? If it does not go well, how will you deal with one or both SBs, or your disappointment
These are probably just my "super planner" paranoia fears manifesting. It's probably why I've never had an SB spring a surprise 3some on me. Probably best to just go with the flow and enjoy.
Like you, I prefer not to tell my SB's about others in my rotation, though I have told some stories about "past" SB's with the knowledge that "past" can be defined as last week. LOL
The last 3some I had was with two porn star SB's, who approached it with the same angle as doing a BGG scene at work; fuck like wild bunnies, then everyone goes home and looks for their next gig.
Have fun tomorrow. The longer it takes you to recover and post an update, the better we will assume it went.
Life is good
The Cat