I got a 2 year multi entry visa 18 months ago for the Schengen zone issued by Germany. It expires some time in December this year. Maximum stay is 90 days and I have already used up 80. I have not been in the Schengen zone in the last 180 days so there is no issue with the 90/180 rule. I am planning another trip to Germany soon, but this should be longer than 10 days.

As other already commented, this is not how your visa works. Any visa valid for more than six months ought to be valid for stays up to 90 days in any 180-day period, not 90 total. That's what the “90” in the “Dauer des Aufenthaltes / length of stay” field means, it refers to the 90/180 rule you mentioned in your question.

It's only if the number of days is lower than 90 than it must be understood the total limit over the whole length of the visa but that's not really supposed to happen for visas valid for more than six months.

So I guess I could cover the first 10 days of the stay with my old visa and would need a new visa for the remaining time. So for example lets assume our trip starts august 1. Then during our stay my old visa would be used up on august 10th, so a new one would have to start august 11th.

My question is if this is possible. My understanding is that you cannot hold two schengen visa for the same time horizon in parallel, but if they gave us a new visa starting 10 days after our intended start of the trip, this would be the case. If they give me a visa starting august 11 and for whatever reason we are not able to depart august 1 then in theory we would have two intersecting visa.

This is mostly relevant for travel around the end of validity of your visa (and not as way to circumvent a length of stay limit) but it is perfectly possible, see Can I travel on back-to-back Schengen visas without leaving and reentering the area?

I got a 2 year multi entry visa 18 months ago for the Schengen zone issued by Germany. It expires some time in December this year. Maximum stay is 90 days and I have already used up 80. I have not been in the Schengen zone in the last 180 days so there is no issue with the 90/180 rule. I am planning another trip to Germany soon, but this should be longer than 10 days.

As other already commented, this is not how your visa works. Any visa valid for more than six months ought to be valid for stays up to 90 days in any 180-day period, not 90 days total. That's what the “90” in the “Dauer des Aufenthaltes / length of stay” field means, it refers to the 90/180 rule you mentioned in your question.

It's only if the number of days is lower than 90 than it must be understood the total limit over the whole length of the visa but that's not really supposed to happen for visas valid for more than six months.

So I guess I could cover the first 10 days of the stay with my old visa and would need a new visa for the remaining time. So for example lets assume our trip starts august 1. Then during our stay my old visa would be used up on august 10th, so a new one would have to start august 11th.

My question is if this is possible. My understanding is that you cannot hold two schengen visa for the same time horizon in parallel, but if they gave us a new visa starting 10 days after our intended start of the trip, this would be the case. If they give me a visa starting august 11 and for whatever reason we are not able to depart august 1 then in theory we would have two intersecting visa.

This is mostly relevant for travel around the end of validity of your visa (and not as way to circumvent a length of stay limit) but it is perfectly possible, see Can I travel on back-to-back Schengen visas without leaving and reentering the area?