Food – little bits more often. If you let them over eat they will get the shits.
Also the more interaction with you feeding them the better. Get ready for them to disappear somewhere and not find them for a day so I would keep any doors shut if they are not already confined to one room and gradually give them more rooms to explore so you don’t lose track of them.
The wood pellet litter tray is less dusty than the granules, see what they take to. At the moment it’s more important they use the litter tray. Newspaper in the bottom, be prepared to start shopping at Waitrose so you can get the fattest paper free each day as well as the free weekend magazine to use in the litter tray.
Socialise, socialise, socialise. And yes, neuter them. The first thing the vet will do is scan them, chances are they might be from cat that has left home to have kittens so they might not be registered.
If you do put 2 trays out you will probably find they both want to use the same one anyway so see how it goes.
Oh and if you have any leather sofas sell them now. Protect any wood they might use as a scratching post and as soon as you know you are keeping them get a good scratching post and teach them to use it.