Suppose we have to fields:

• A and B,
• for both fields there is registered OnChange() event,
• field B is disabled.

In OnChange_fieldA() code we are changing fieldB attribute, but OnChange_fieldB() is not fired. Why?

Build-in javascript mechanism doesn’t notify field attribute change if field is disabled or change is made via JS code, not via UI.
In that case can force field to fire it’s event:

Xrm.Page.getAttribute(“CRMFieldSchemaName”).fireOnChange();