I haven't found an answer for this question.

Suppose I have a database table with dozens of fields and several related tables.

Can I create two or more entities for it with different set of fields (and collections of related objects in child tables)?

Maybe I asked incorrect question?

Yes, you can.

I mean maybe my question about several entities for one table was incorrect in terms of NHibernate idea.

In general, I would recommend doing it, since it may have issues with concurrency, with different views of the same data, etc.

You need to consider the case and decide if this is possible or not.
I would usually try to refactor the DB first.

I think it is not a rare case to have a different views of the same data.

E.g. I can have two versions of the same object, a "heavy" version with all properties and all related collections of child objects and a "light" version with some set of properties. Also I think that these "light" objects are often read-only.

For read only data, I would agree.
I am concerned with multiply updated views of the same row, especially if some of the columns are shared and updatable.