Binding Multiple Actions to the Same Key

[CSargeP]

I'm trying to bind multiple actions to one key.

 

In this screenshot you can see that by default multiple actions are set to Mouse 0 (left click) but when I try to do something similar like adding panning the camera to left click I only have the option to unbind everything else.

Is there something I'm missing or a button I need to hold down to let me bind multiple actions to the same key?

 

Thanks in advance!