How does fee split between senders in a transaction?
There is no real concept of sender identities, as far as the blockchain is concerned. The fee split is entirely determined by the change output values, which are entirely determined by the signers of the transaction.

A transaction involving inputs from two separate parties will require some coordination between them to decide on the recipient and change. It is presumed that during this coordination, a fee is decided upon, and set.

Bitcoin itself offers no insight into this splitting process, it should be handled out of band by the wallet or tool used to construct the transaction, based on configuration from the senders