How do i normalize diff information (different type of documents) in the index ?

[s d]

e.g. if the index is field1 and field2 and documents of type (A) always have
information for field1 AND information for field2 while document of type (B)
always have information for field1 but NEVER information for field2.
The problem is that the formula will sum field1 and field2 hence skewing in
favour of documents of type (A).
If i combine the 2 fields into 1 field (in an attempt to normalize) i will
obviously skew the statistics.
Please advise,
Thanks,