You must have both registers (or all registers needed for the group) to be assigned to the same "Register Group".
manager's menu > register setup > setup configuration > revenue centers. Type a name for the register group at the top and click save.
Manager's menu > register setup > paths terminal setup. At the top, select the correct register group in the drop-down menu at the top of the screen and click save. Do not choose to update all terminals when prompted.