Servos drives directly start turning

No, the servo drives of the Y-Gantry each have their own connection to the encoder TB3/4 and analog interface TB5.
Attached is a picture of my 7i77 axis connection.

If I connect only one of the two Y axes to the analog interface TB5 and enable with F2 the axis enters control mode (drifting slightly but no jog possible). However, if I connect both axes to TB5 and enable them with F2, the axes immediately spin at high speed and a following error occurs.

@PCW What do you mean with "test  this with the linear axis disconnected"?

Note: The axes are currently not connected to the mechanics and are essentially lying loose on the table.


 

Encoder issue? Did you that  both encoders work?
Are they Scaled properly and in the correct direction?

Why does one joint have the scale reversed?

If the axis drifts when enabled, encoder feedback --> PID --> analog output  is not working

If the axis runs away when enabled you most likely have the feedback loop reversed

Unless both motors hold position when enable (and resist applied torque)
the feedback loop is not working and needs to be checked on part at a time