Faceting question

[Mark]

Is there anyway to perform a search that searches across 2 fields yet 
only gives me facets accounts for documents matching 1 field?

For example
If I have fields A & B and I perform a search across I would like to 
match my query across either of these two fields. I would then like 
facet counts for how many documents matched in field A only.

Can this accomplished? If not out of the box what classes should I look 
into to create this myself?

Thanks