Can i have security profile with 2 OUs assigned to a responsibility, wherein both OUs belong to different Business groups. I know it is technically possible by creating Global security profile. But the question is ...how it works when i try to create a PO/Invoice with that responsibility ? since employee is attached to a business group how i am going to create PO/Invoice in other OU/Business group.

I understand, we can have these kind of responsibities only for reporting purpose. am i right ?

This is an intended functionality by Oracle .
Single responsibility could have access to OUs that roll up to multiple business groups.

When a document (like invoice etc) is created, it is created in OU, not in BG. BG only determines available employees list.

Profile option 'HR:Cross Business Group' plays an important role.
When profile option 'HR:Cross Business Group' is set to No, all employee LOVs will be restricted to the business group defined in the 'HR:Business Group' profile option irrespective of the OU context.