G'day Jovimon
Say one is called "X" and the other "A"
Slave them together using the menu item under "Config"
You have probably already done this.
Then go to "Homing/Limits" also under the "Config" menu, and select "Reverse" for the appropriate one.
That will do it.
This presumes you are using LPT.
If using homing switches you can have one for each motor and Mach will home each one individually.
Handy on say a gantry axis where it will confirm the gantry squareness when homed.
If using only one home switch for both slaved motors then there is a selection under "Gerneral Config"
to "Home Slave With Master Axis" and Mach will home them as a slaved pair.
Greg