The condo by laws and covenants should be reviewed to determine the answer. In the scemario you are describing it would be very hard to assume that only one party is responsible. If both are responsible and you know of an actual problem with the system (you've just done a septic inspection prior to closing), then make sure the seller of the side you are interested in resolves the issue prior to your purchase. Both parties olikely need to agree on it's failure and create a mutual solution. You will inherit the problem.
A related question is: How is the capital reserve fund set up and is there enough money to repair the issue (roof, septic, painting, etc)? Smaller 2 unit condos tend to have looser agreements (and don't follow the written bylaws). The positive is you just get together as a neighbor, get a quote, and split the cost. The negative is if the money doesn't exist from the other side, you may have to foot the bill for a new roof and get paid later. Hope that helps.
Good fences (bylaws and money in the slush fund) make good neighbors in this case.