I think this is a general skill, outlook, attitude about about intersecting with people in general. Although it has specific applicability to relationships (your context), it has a lot of crossover into a variety of parts of life.
When we intersect with people, not many of them are existing in a vacuum. They have some existing life (hopefully) at the moment and there's going to be some challenge to figure out where our place might fit into that.
I think if your attitude in general is that you are going to react with other people and sweep them along into your world, your life, your culture then there's going to be difficulties. The important part at first is learning about each other - what our various worlds look like. Then we can decide how they might mix and how they might conflict.
Relationship-wise, I think anyone who is 'single' is going to have more difficulty coming to grips with this as it's not their current mode. If they have some experience from the past you would hope they might be able to 'switch gears' more quickly as they've been there. They just need a reminder of what it was like. But if they have little or no relationship - or as you term it - "merged" experience, then there's going to be some learning curve and they have to be up for it.
Thoughts at first anyway............