'B, I'm happy to hand over your ticket if you'd like to collect it, or pay for recorded post - I can do either - or if you're happy to risk regular post, I'll drop it in the post box tomorrow.
I'm selling A's ticket as she owes me £400. If you don't want to go without A this is also fine. If I find a buyer for your ticket I can just refund you.
Let me know if you still want your ticket, & if so how you'd like to receive it, or if you'd rather I see if I can sell it on - if I do manage to offload it of course I will refund you.'
Then sell A's ticket to your other friend & enjoy the gig. If B wants her ticket, ok, she's paid for it, but you've offered ways she can get it so you've done your bit to see she isn't out of pocket. Up to her to tell you which suits her.
If you don't get a sensible reply, just ignore them both. See if you & your other friend can find a third to take B's ticket; if so, refund B.
Otherwise, B can swivel - you offered it to her, she didn't take you up on your suggested solutions, so oh dear, she chose not to collect it, not your problem.
& block both the day of the concert. You're done.