Motorhome leisure batteries

I have recently bought a secondhand 2010 Bessacarr E520 which has been fitted with two leisure batteries, one of 75AH, the other 90AH. There were no apparent problems, but the company which has just carried out the habitation check has told me that this is not acceptable and has disconnected the 75AH one, saying that they must be equal or damage to the charger will result.

I have had advice from another dealer who says that provided there is a fuse between the two there is no problem! I am confused, and wonder whether anyone has any advice to offer?

Many thanks,

Steve