How can coinbase transactions be created? How can a coinbase transaction be created and what would happen if there was more than one coinbase transaction in a block?
The current bitcoin system does not allow to create a block with two coinbase transactions. 
The rules are described here. 
So a player against the system would need to craft such a block manually, which would get rejected, and eventually the node would get banned.

Hint: as his is a basic question of how bitcoin works, I recommend to read the technical articles on bitcoin.org, as well as the article on protocol specification in the link. And on top Andreas‘ book „Mastering Bitcoin“ gives a deep insight into the bitcoin world. It is also online available, in many languages. Highly recommended!