Though it may seem a good idea, registering your pet on multiple databases does not make your pet more likely to be found in the event that they go missing. Having to check with multiple databases can hinder authorities while attempting to contact you as the keeper, as they will need to contact each database with a registration to try to figure out which has the most up to date information.
Registering a single microchip on multiple databases can prevent proper keepership checks, making it difficult to verify if an animal is reported missing or stolen. This loophole can also allow someone who has stolen a pet to appear compliant with the law by registering the pet in their name.
Most microchip databases in the UK will not register an already registered microchip as best practice and will communicate between each other if keepers want to transfer microchips between databases.