It's theoretically OK to run as many CU's in a property as you want, the issue is about clarity and ease of operation and very often less is more I tend to find. If you DO fit a separate board, make sure it's clearly marked, you'll need to have clear indication of where the isolation for it is, and also on the existing board too. You'll also have to fit ONE separate overall main switch BEFORE the tails get split in order to satisfy the requirements for a single point of isolation in a domestic setting. Also of course you need to make sure your earthing is all bombproof, it's not just about the tails.
Generally speaking, by the time you've messed about with all the above it's quicker, easier and cheaper just to fit a new bigger board for the whole place. Shiny shiny. I normally find in kitchens there's a million and one gremlins hidden behind the tiles, so take your time and 'think twice - cut once'!!!