If, for instance, the left side of your single digital market has a copyright duration of 70 years after the death of the artist, and the right side has 90 years instead, then in effect, it's 70 years even on the right side because anyone can just go to the left side, trade copies of the expired works without paying licensing fees, then come back to the right side with those copies (because it's a single market).
So in effect, the regulation of the whole market is the weakest regulation of any part of it.
If, for instance, the left side of your single digital market has a copyright duration of 70 years after the death of the artist, and the right side has 90 years instead, then in effect, it's 70 years even on the right side because anyone can just go to the left side, trade copies of the expired works without paying licensing fees, then come back to the right side with those copies (because it's a single market).
So in effect, the regulation of the whole market is the weakest regulation of any part of it.