What causes a two headed mutation or is this a malformation?

It's a mutation

Explanation:

Yes carrot

It is also possible for two separated embryos to incompletely fuse to form a two-headed animal. Two-headedness results from factors that cause malformation of a developing embryo, which can be both genetic or environmental ... A two-headed bullshark caught in waters off the Florida Keys in the US

Explanation:

Its a mutation