From the 7252/7252i manual:
This option is only available with instruments where both channels have a full scale pressure range that is greater than 10 psi (69 kPa) and less than or equal to 1000 psi (6.9 MPa).
The option must be enabled (set to User) in the factory menu. To check this in factory menu follow this menu path and set "Test Ports" to "User":
Menu>Menu>Setup>Limits>Access>9750547
Test Ports: User/Tied/Untied
Go back the the main screen by pressing the [previous] button until back to the screen. Then follow this menu path and set "Test ports connected" to yes:
Menu>Menu>Setup>System>Config
Test ports connected: yes/no
The remote command to detect state is DIAG:SYST:COUNT? It returns the number of active channels. 1 for test ports connected (no B channel), and 2 for separate A and B channels.
7252i controllers 1000 psi and less leave the factory with factory menu setting “User”.