I have following situation:
2 datasources should be indexed by Apache Solr.
CSV file just update 2 fields on already existing documents in Solr.
But I have following problem: it seems impossible to make update just some subset of fields by DataImportHandler. Is it really impossible or there is some trick?
I believe your problem lies in trying to "update" an existing document with only two fields. Which version of Solr are you using? Partial updates are only supported for Solr 4.x.
Here's some information on what partial updates are and how to do them: http://wiki.apache.org/solr/Atomic_Updates
http://solr.pl/en/2012/07/09/solr-4-0-partial-documents-update/