This question goes one step beyond the SB threesome scenario. It's beyond my experience, but here's some thoughts...
From what little I know of swinger parties, I think you are best not disclosing that your partner is an SB, just like bringing an escort is frowned upon (but done from time to time based on the providers I've seen on TER who advertise it in their ads)? Am I correct?
If you have gone to "lifestyle" parties in the past, can we assume you are already comfortable having sex in view of others, including other men and women who are (I presume) relatively anonymous strangers? Will it make a difference if all four of you know each other outside the bedroom?
Are you only looking to swap partners, or just be in the same room? Are you and your SB Bi? Do you want to explore more than just a straight swap (Ex: MFM, FMF, MM, FF)? Do you need to agree on "pre-BCD" rules like test results, sex practices and acts, role playing, boundaries and limits? Do you need to agree to any "after-BCD" rules? For example, can you date his SB without him in the future? Can your SB date him? Can he date both SB's without you and vice versa?
How will you handle allowance? Will each SD be responsible for covering allowance for their own SB? Will your SB be ok with not getting additional allowance, either from you or the other SD, if she is expected to go BCD with you and the other SD? What if there is a huge disparity in allowances and one SD is typically providing 2x, 3x, or even 4x the allowance provided by the other SD? Will both SB's be aware of allowance amounts?
I'm not trying to dissuade you from moving forward. I just suggest you may want to consider these topics in detail; first by yourself, next with a prospective brother SD, and then each SD with their SB's. It also seems prudent to do a non-BCD M&G first, so that you can ensure compatibility before getting naked. How awful would this go if the SB's end up disliking, even hating each other, or the "other" SD? Hmm... how awful would it be if his SB turned out to be one of your ex-SB's or a POT you rejected in the past?
If you manage to make this happen, without anyone's pink bits getting cut off or kicked, you will be blazing a new path in the Sugar Bowl.
Ride on, Parder! Blaze that path!
Life is good
The Cat