Advice please on room only lettings

Tenancy Deposits

Hi
I have an hmo which is not licenced ( not required to be licenced). It has 3 letting rooms and i havelet to single people for the past year. I now have an enquiry from a couple for a double room. I think legally this is ok as 4 people and licencing if 5 people. The rooms are let including bills
But I'm not sure how i would register the deposit ..... do i split the deposit and half for each of the couple?
And is it reasonable to ask for more rent if it is let to a couple rather than single person. The property would suffer more wear and tear..... and potentially higher bills if people were in at different times, and more water and electric usage.
My main portfolio is to let entire houses and i'm not as sure of protocols on room only letting.
Please can you advise