This is happening because the dosing pump numbering for the 2nd dosing unit has not been assigned.
Follow these steps:
NOTE: This applies ONLY to first generation GHL dosing pumps.
1. Disconnect the 1st dosing unit from the S-port on the P3
2. Connect the 2nd dosing unit to this same S-port on the P3
3. Connect to your P3, go to the Switch channels settings page, click Program digital powerbar or dosing unit
4. Select Numbering of dosing pump station
5. Set this doing pump number to start at 5
6. Click Next, then click Program Now
7. Disconnect this dosing unit from the S-port and reconnect your 1st dosing unit
8. Connect the 2nd dosing unit to the port on the back of the 1st dosing unit
FYI
If your "Program dig.powerbar and dosing unit" is not clickable, you must enable digital communication on the port where your dosing units will be connected to. If on S1-S4, enable that port. If on S5-S8, enable the port. Save settings.