Yes you can, but you wont be able to run both machines at the same time - not without some clever Gcode. Would be a fun project to try that actually...

Anyway, if it's one machine at a time then you could use the same stepper drivers and just plug the motors from the lathe/mill in depending on which you're going to use. That also saves you a lot of money in stepper drivers!

Alernatively you can plug all 5 drivers (assuming 3 axis mill and 2 axis lathe) in to the computer and easily set mach3 to use the correct ones depending on if you have it in lathe or mill mode. There's lots of ways of doing it - you could have a physical switch to switch between motors or drivers.