So, imagine the situation, when we have such data: one part of it is better to store in RDBMS, and another one in some of NoSQL (Mongo, f.e.). Is it correct to use 2 separate DB? Or should I find a some compromise for data structure to use only one?
I guess, when we use 2 DB we need much more resources (and carry more about fault tolerance), but we can have better stucture. Or my way of thinking is totally wrong? Never heard about this kind of practice, so want to ask more experienced people.
Btw, sorry if my english is bad.
Using different dbms for different kind of data and purposes in the same application is definitely legit. It does come to some cost as you mentioned, but if the benefits outweigh the costs I would go for it.
A not so uncommon usecase would be to have a rdbms as persistent datastorage and add Lucene as fulltext search engine. So you can use the rdbms for your acid compliant operations, to enforce constraints.
In the next moment you can use lucene to search for some words, get the corresponding identifier and then ask the rdbms for more information belonging to the identifier.
If you check the 'more exotic' NoSQL databases (graph databases, column based databases) you propably will find more such usecases in which 2 dbms are great to supplement each other.