What you need to locate is a copy of the Vanagon Type 2 / Fuel System
Pro Training Manual.

It cover's the dual switch Throtle Positioning switch arrangement, (as
well as the single) really well, and has an excelent diagnostic section.

You'll be scratching your head for months reading that Bentley.

If your idle switch isn't se up right, or it's just plain worn out,
you'll never get the engine to run right coming off idle.

If the Full throttle swich is Kaput, r not adjusted right the ADM will
never know when the throttle is opened or closed.

You need a vacuum gauge and a hose to check the vacuum.
You'll need a digital mutimeter to check if the switches are working at
the right time ( or at all ).
You'll need the right book to get everything ironed out.