It is not per se. But running brothels is.
Two girls working from the same property constitutes a brothel, thereby making it illegal and the law needs to accommodate this.
But you said prostitution, it is not illegal to be a prostitute, there are lots of other things are, including shooting someone, but the point you made was about prostitution.
The reason pro$$ies don't declare to the tax man is simple, they don't want to pay tax or lose benefits, and as the transactions are cash they can hide the trade.
They would not like to work in a regulated brothel with health checks, tax and charges, and loss of benefit, that would be like turkeys voting for Christmas
As for changing th elaw to allow girls to flat share, why? what benefit does that give other than increasing the girls profit by lowering her costs, ok if you want a threesome could be handy.
You will hear pro$$ies say it's for "security" but that';s just a smoke screen as a size 8 5ft cutie is nowehre near as effective as security as a 18 stone 6ft 4 guy, but then they would have to pay the guy