I put a 2nd tank on my Syncro Westy in '01 and connected them.
I use one and know the other is full, so I never run out when
camping.

http://jones.colorado.edu/VW/SecondPropaneTank.html

My impression is that in Europe tanks are stored inside in
a cupboard.  What I don't know is whether they are connected
or you have to take them out and connect them when camping.

Richard