How do I split the screen of unity as in the old first person shooter?

'old'? Old what? 'The old first person shooter' Did it have a name?

im posting this as a comment because its the only way i can think to do it off the top of my head, Im sure there must be a more efficient method

but you could render the players camera feeds to a movie texture and just put the texture planes on top of eachother in front of the main camera