Hi all - We are looking into an owner occupant property.  The property is roughly 1 Acre with 2 separate houses and 1 shared Septic tank.  Primary house has (4 Beds, 3 Baths), Secondary house (rental) has (2 Beds, 2 Baths)

From what we know - the waste from Secondary house goes into basement of Primary house and then gets pumped out to the Septic tank.  The Septic system was installed in 1984 and it overflew in ~1989 (35 Years old).  Since then we didn't find any bad history about that.  Based on 2012 laws, I have read that two houses cannot share a septic tank, however, it seems this MIGHT have been grandfathered in.  
What are some things we must find out prior to closing?  What are few things you recommend?  We are booking a Septic Inspection next week.  

Thank you all in advance for your help!