I started a long email explaining why I'd chosen that configuration. There needed to be a run up to conversation A, and you couldn't have A and B right next to each other, so X A X B was the absolutely perfect order. As I wrote my justification, I thought as a concession I'd try X A B, but that obviously didn't work. I tried A B X - no, it definitely needed the X in-between. A X B was on the surface the straightforward choice, but that would mean rewriting the intro, rewriting the X interchange, writing a completely new run up to the A conversation.
As I wrote a length email to my editor explaining why my first choice X A X B had been the right one, I realised: I didn't want to change the order simply because it meant more work. After a short bout of internal wrestling I deleted the email and wrote another, shorter one. You're quite right, I wrote to my editor. I'll do it.
That's what an editor should do - poke/prompt/nudge/direct you into writing better. I did all the work (which actually was very enjoyable once I'd decided to go for it) and the book now starts intro then A X B. And it's much, much, better for it.