Sharing always has drawbacks. I am not into sharing or ntr but one day I was strolling through a porn site and saw a couple streaming together, then I was like okay good for them. Next stream the guy invited his friend and still was like okay good for them. But during said stream, you could see she felt a hell of a lot better with the new guy and didn't want to do anything with her boyfriend only his friend. The next stream chick left her boyfriend for the guy's friend and started streaming together while she dumped her boyfriend.
Shit a guy on the porn site even broke the video down and explained in the key moments she changed. Felt like I was watching a youtube video and not porn.
In games or videos, it is fine since it doesn't affect me but never would I share.
So do I think they are the same thing? Yes and no. While sharing isn't the same thing it can become it if you aren't better than the person you are sharing with. And this goes both ways for both males and females.
I know your post is from a few months ago, but I just saw it, and it reminded me of something I saw once from the opposite side, so to speak. I once saw a porn video that I didn't know was sharing until after it had started. It was like watching a train wreck and I couldn't look away, I don't think I could have gotten hard watching it if my life depended on it, and it honestly put me off porn in general for a long time. The chick kept trying to get her husband involved, talking to him, and even at one point when he was close to her she reached for him and he literally pushed her away. You could tell she was just doing it for him. Her husband didn't even get hard until the other guy came inside her and then the husband basically took three pumps in the sloppy seconds and came too. Imagine being a woman who loves a guy and the only way he can even get hard is when some other guy bangs you first and he won't even let you touch him until the other guy is done.
So for me that was a bit of a defining moment when it came to my understanding of what "sharing" really was.
NTR is considered by most non-Japanese people to be when a slut cheats on a guy. Though in Japan a guy could be walking home from work and turn down an alley and see a random whore getting fucked, then take another route and see it again, repeat a few times, and the video would be called, "I got NTRed over and over on the way home from work," because there it can just be seeing (or even hearing about) any two other people fucking.
Sharing is one of two things. Either when a slut convince her man to let someone else fuck her because she wants to cheat but wants permission first and that almost always ends up how you're example went, because women cannot equally respect two men (and if she respected her first man she wouldn't even bring it up) and one will be "better" than the other. The internet is filled with "my wife's boyfriend" stories where the other guy is clearly the one she cares about and the former boyfriend/husband is treated like a third wheel at best and often as a man-child that the wife/girlfriend and her new guy just tolerate because he pays the bills or something but neither of them respects.
The other type of sharing is like what I saw, a women who is basically being forced into it out of devotion to a piece of shit guy who doesn't really care about her and is using her like a trading card for his sexual enjoyment and doesn't give a shit if she is really into it or not and usually it turns out the guy is gay or otherwise ends up leaving her anyway, or she finally gets that he isn't worth it and leaves him.
So basically if the woman wants it then she's just a whore and it is NTR with an extra step because she doesn't care about her starting man just didn't want to "cheat." Or the woman is basically being forced into it and essentially being "raped" by men she wouldn't fuck if the man she does care about wasn't making her do it and it will eventually destroy her and their relationship.
So are NTR and sharing the same, well, "no," but in every way that matters, yes, they are the same, they are built on the same emotions as a foundation and eventually end up in the same place. That's my two-cents at least.