Whether you want to combine a bunch of lists of locals or see who fifteen different food trucks are mutually followed by, you can do it with Formulists’ “Custom-Lists”. Using “Custom-Lists” you can merge, subtract or intersect up to twenty different twitter lists with one another.  Just sign in, select the “Customize Existing Lists” category and click to add a “custom-list”.  

From there you can add, subtract or intersect existing Twitter lists like so:

In the above example, two lists are being merged.  However, by clicking on where it says “add people from this list” you will also see the options to instead “remove people from this list” or “keep only people from this list”. 

And as a warning, if you want to combine lists of yours or others that you expect to be deleted, make sure to turn off the Formulists automatic updates.  Otherwise, when the list is deleted, the members of it will also be deleted from your “custom-list” combo.