Splitting a tablet should only ever be done upon the advice of a doctor or pharmacist.
Splitting antibiotic tablets, for example, so that you receive a lower dose, might result in you receiving an insufficient amount to deal with the infection while, at the same time, building up a resistance to antibiotics in your body, so that they'll work less well in the future.
Splitting coated tablets is also something that you should never do, as the coating is there to ensure that the drug enters your body at a controlled rate. With the coating removed where the cut is, the drug will be absorbed at a faster rate than intended.
A relevant link:
https://www.webmd.com/a-to-z-guides/news/20110107/splitting-pills-may-have-risks