Indeed, my spouse is the same way with their other partner. I am friends on facebook with Other Partner and sometimes i'll tell them when we have made plans for a certain date so that they can plan around it. It isn't meant to mean "I get first dibs and you need to make do with what's left over." as far as i am concerned, Other Partner could make plans and let me know what they are as a courtesy the same way. it embarrasses me to some extent when Other Partner says "i am happy to spend whatever time i can with [your]Spouse" because it reminds me that they think of themself as "second" in priority, like a scavenger bird that waits while the "real" predator finishes eating the carcass in order to have their own dinner. While that's fine for vultures and hyaenas because that's the role of those species in the food chain, we are human beings and i don't think of our importance in [my]Spouse's life as diminished by the reality that we are both present in it. Of course, Spouse is an extraordinary individual who is held in high esteem by everyone i know of who comes into contact with them. However, Other Partner has no previous experience with "sharing" and is trying to not offend or alienate me. I just don't want them to think they have to walk on eggshells or sell their own needs short out of fear or ignorance.
Oh well. It's getting a little hijacky and i'm trying to watch out that i don't do that too much. It has been brought to my attention that i have a tendency to hijack, and i rub folks the wrong way even without intending to do so.