RP, I feel like I totally understand what you're getting at by the idea of 'merging', which I think is largely due to my tagging along on the threads about primary/secondary labels for the past few months. I'm appreciative of and interested in your choice not to use those labels here and get at underlying issues-- and
I think referencing back to the shorthand of 'primary' might make some of this discussion more straight forward.
I'd like to elaborate a bit on other kinds of merging that happen no matter the relationships people have... There are people who are married to their jobs, and that commitment is one that a partner needs to take on willingly-- marrying an ER surgeon or a firefighter means that your spouse is going to need to leave abruptly often, albeit with the best of reasons, and you both have to find ways to make that work within the relationship. My job is nowhere near that strenuous, and between work and the commute I'm still not home from 7am to 7pm, five days a week. How do we make time for each other?
Then there's people with passionate hobbies: someone who has no interest in sports dating an enthusiast needs to figure out how to cope with it, and they both need to make decisions about how to fairly negotiate their time together. Same goes for historical reenactors, shoe moguls, and craft enthusiasts... Then there's people who insist on alone time, or on having three date nights a week with other people
In the end, I feel like it all comes down to time management and prioritizing. "This weekend, I'm going to a LARP, and next weekend I'm with my other boyfriend, so on Tuesday and Thursday you and I should go out for coffee." We all juggle it, and we find ways to make it work.