@Neil408 I feel for you, & hope it's resolved in your favour.
-It's difficult isn't it, when two turn up having only paid for one.... Trouble is, Airbnb don't make it easy to recuperate the money through the system. The first time it happened to me, I TRIED to go to the 'change reservation' page, and asked the offending guest to do the same, but we both got a flash that you can't amend the booking once the reservation has started, i.e., after earliest check in time. I was in a panic, so I asked for cash, (only £10) not knowing what else to do, and because the guest was arguing the piddling £21 he'd paid for the room FOR ONE should suffice! - Thankfully he paid - I later spotted he paid with one legal plastic fiver & one illegal paper fiver, no longer legal tender! I left him a rotten review for this, & for leaving s**t all over the loo shared with me & other guests, for taking a mirror off the wall in another room, & putting it in his, & for checking out 1 hour late at 1pm. THANKFULLY, he did not retaliate by informing Airbnb, nor did he write a review. I NOW KNOW I SHOULD HAVE ASKED FOR THE SECOND PERSON FEE THRU THE RESOLUTION CENTRE, (different from 'change booking'), & got him to click the necessary acceptance BEFORE I let his wife up my stairs! I will do this next time!
These sort of guests put us in a vulnerable position, cos they make US look like villains when we are stalling them in the hallway, denying access to the bedrooms till they've paid for person Number 2! A host near me lost her 100% 5* reviews when a guest said she was upset by an unwelcoming arrival - all because the poor host wished to phone Airbnb about the 2nd person not paid for before allowing up the stairs!
I must say when I now get a booking for one which is not a work visit, I check via messaging that there's only one.... so if two, they can hit the 'change res' button BEFORE check in....
My only advice is to grovel to Airbnb, say you are sorry & panicked, & didn't understand what to do, as the reservation had started, so the 'change reservation' button would not activate. Maybe say your first concern was not to upset the guests, so you accepted their cash offer, so as not to seem a confrontational jobsworth; you were just trying to be a nice host!
(Your description of the girls reminds me of several threads in the archives here, lamenting young Chinese females who travel in pairs! (Late night showering, flooded bathrooms, & washing their smalls - an amusing read, even if your ladies were not Chinese!)