One is the Kustom MLI monitor speaker.
It has an impedance switch to select 8 or 16 ohms.
http://www.kustom.com/products/pa/switchable_impedance.asp

The other is the Accugroove bass cabinet - it has a switch to select either 4 or 8 ohms. (But I found a thread on talkbass and apparently the switch doesn't measure up to its claims.)

My confusion is how a switch could select just double the impedance. For example: 4-or-8ohms, or 8-or-16 ohms. I can't see how you can series/parallel to get such combinations.

It'd sure be handy if such a switch were possible.