Slacker, I think my perspective may be of BENEFIT to you.
Har har har.
Certainly spending more time working on the relationship would strengthen it! Kind of like the sports motto: "Hard work beats talent, if the talent fails to work hard." --> Hard work beats natural compatibility, if the natural compatibility fails to work hard.
Maintaining any kind of relationship takes some amount of time, and effort. Generally, duality just takes less time than others. But certainly, if you put more time in, the relationship would get stronger, no matter what kind of natural compatibility exists. And non-duals putting in more time, would find the relationship stronger and more fulfilling, then duals who don't.
In the duality description, it says, maintaining a dual relationship is less time-consuming, so that each person may spend more time on their own interesting activities. Do you want to spend more time on your own activities, or more time into the relationship? Do you want a stronger relationship? What do you want? Are you just speculating why non-dual relationships are stronger? If the last question is what you're looking for, it's simply because they've put more time into it.