From a purely mechanical standpoint, if you are using ACME lead screws you can use a right hand thread on one side and a left hand thread lead screw on the other side of your machine and the steppers can run in the same rotational direction. Left and right hand thread ACME screws are available from McMaster-Carr.
There has been discussions of how to connect two steppers to one driver on CNC Zone in the past but I don't recall seeing anyone do it successfully. Using the right search terms in the forum search box may find some useful information. It isn't as simple as paralleling the motor wires on the same terminals. Most low cost drivers would not have output circuits that can survive it. It will be best to use a 4th TB6560 driver.
CarveOne
http://www.carveonecncwoodcraft.com