Can anyone tell me (or point me at information) about
how statistics are generated/used for multi-column indexes?

We have a 2 column status model (major and minor status, if you like).
This is key to our application.

"Naturally" we have put a 2 column index on these two columns.

But we have "doubts" about whether the CBO is always
doing as good as job as we'd like.

The key question seems to be:

Are the columns "treated as a pair" so that the
frequency estimate for a pair of statuses depends
on both of them or...

Are the columns treated in isolation
so that the frequency estimate
is simply obtained by getting the estimate
for each column, and combining them mathematically.

This would make a big difference in our app,
since our major/minor status (in practice) have interesting
and complex correlations.

BugBear