1) 5.6 - 6.0 upgrade was available for 6.0.0003 version and it deletes fields of second person. So it will need modification to keep second persons and upgrade to 6.0.0004, 0005 also.
2) For upgrade you should convert second persons to profiles and make bound between their first persons.
You should make custom upgrade anyway. Because:
1) 5.6 - 6.0 upgrade was available for 6.0.0003 version and it deletes fields of second person. So it will need modification to keep second persons and upgrade to 6.0.0004, 0005 also.
2) For upgrade you should convert second persons to profiles and make bound between their first persons.
Regards