#Japt -h, 11 bytes

à f@øX «VøX

Try it

 :Implicit input of strings U & V
à :All combinations of U
 f@ :Filter each as X
 øX : Does U contain X?
 « : Logical AND with the negation of
 VøX : Does V contain X?
 :Implicit output of last element

Japt -h, 11 bytes

à f@øX «VøX

Try it

 :Implicit input of strings U & V
à :All combinations of U
 f@ :Filter each as X
 øX : Does U contain X?
 « : Logical AND with the negation of
 VøX : Does V contain X?
 :Implicit output of last element