Let's start with some definitions. The property that gets to use the easement is called the dominant tenament or tenant. The person who has the easement on their property is the servient tenament.
A sewer easement is quite common and they are usually non-exclusive. That means the property owner can use the property, and even grant additional easements so long as they do not interfere with the use of your easement.
You are responsible for ordinary maintenance. If the pipe brakes, or needs to be replaced, its on you. The servient tenamtant cannot do anything to interfere with your use. If they do, they pay to repair. If they are out digging a trench and the break your pipe, they have to fix it.
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I am not clear on the interrelations ship of the two easements, but the starting point would be to read the easement agreement(s). The document may contains specific provisions about liability, maintenance and other things.
Absent something in the agreement, Oregon law allows the property owner to use the property over and around your easement (the "non-exclusive" concept), but the other property owner may not unreasonably interfere with the easement. If the other property owner causes damage, most likely it would be his responsibility.
On the other hand, you would most likely have a duty to maintain the line, and if it shows signs of leakage or deterioration, you would need to repair it. I don't think, in most circumstances, you would have liability for an unanticipated catastrophic failure.
This comment is general in nature and is not intended as legal advice. It does not create an attorney client relationship and obviously is not confidential. You should contact an attorney in your area who can review with you all of the relevant facts and give you specific legal advice.