disabled in interface

I have a doubt in the parameter "Disabled". 

I have a form with some fields. For one of the field (Customer) I can select values via 2 options ( one from the dropdown and other is the text field) . These values are stored in dropdownCustomer and textCustomer field. 

I want to disabled the button if some fields are blank in my form. My doubt here is, for the customer field, there will be only one value present either the dropdown or the text field. 

If I check a!isnullorempty(dropdownCustomer), it maybe null but the other one textCustomer might hold the value. so, I need to check if any one of the field (dropdownCustomer or textCustomer) has a value, then I don't want to disbale it. Can someone please help here. 

disabled:

or(

a!isnullorempty(field1),

a!isnullorempty(field2),

a!isnullorempty(dropdownCustomer)

)