Hi Ant. Welcome to UKAPS. Anubias have a rhizome, which is the thick root that the leaves grow off. If you have a reasonable amount of rhizome, a good few centimetres, you can cut it, but you need to use something sharp to make sure it's a clean cut.
Also, anubias needs to be tied to rocks or wood, and not buried in the gravel like most other plants. If you bury it, the rhizome will most likely rot and kill the plant.