Posted for George Braly <gwbraly@gami.com>:
Jeff,

Good comments.  Let me add some experience.

I used to have 13 lbs worth of back up electric powered vacuum pumps in my
Bonanza.

One day, the main vac pump failed.  Within 30 minutes,  the attitude
indicator quit, even though the backup pump was working.  Why?  carbon crud
sent through the attitude indicator by a poorly thought out filter/switch
over system.

So... out went the 13 lbs of back up pump and electric motor.   Installed a
3" electric AI where the old 3" T & B was located.  Installed a 2.25"  T & B
where the old 2.25" clock was.

I have, since, had two vac AI failures and one electric AI failure,  and in
each case, it was fairly easy to figure out which was working and which one
was not.  Finished the IFR flight in one case.  The other two were VFR.

John Deakin (writes for AVweb) had the same double vacuum pump failure in
his Bonanza.  It is not an uncommon failure mode.  But then he is cursed, as
he once had a double mag failure... but fortunately, it was on the ground!

Regards,  George