> Is there any way to modify a standard, single sliding door to a double?

Sure. First, get a right-hand drive Transporter (or at least all the
necessary door parts), do a little cutting and welding, and voila...
They all started off essentially "ambidextrous", as far as I've seen,
and the lefty-, righty-, or bothy-ness happened with the addition of
the appropriate panels, rails, guides, etc. at the factory.