Jim Weir wrote...

The question is whether to have a two-channel meter
(which complexes up both the design AND the panel space), a
single meter switchable between lateral and vertical) or
a single meter with the two axes summed together.

You gotta decide what you're going to do with it and what
it's going to tell you in the cockpit. Are your procedures
going to be different if one axis reads high and the
other reads normal ("Holy cow, lateral vibes are off scale!
Good thing it's not vertical!")? Personally I'd think that
total vibration (RSS) would be sufficient to indicate a problem
that warranted more detailed troubleshooting on the ground.
Too much information in the cockpit can be just as bad as
not enough.

Dave 'standard gauge' Hyde