Swinging just means both or a single person in a relationship engage in sexual activities with others, sharing is 100% encompassed within that, the only difference between the two is that sharing also includes sharing someone who isn't your significant other. (E.G. picking up some chick at a club while out with friends and end up having a threesome with her and one of your friends.)
So while they aren't 100% the same, sharing is still included within the bounds of what swinging is. Is it worth making a separate tag for it just for that... well Netorase will be a tag soon, which WILL likely be what is used for any form of sharing from now on. I think it's way more important that the NTR tags are finally being separated, arguing about swinging vs sharing is kind of silly.