I spoke to Anet Support today as I requested some details about Eternity owners and how that will work with the armory. For some backstory, I crafted and bound Sunrise back in 2015, and crafted Twilight to create my Eternity on 7/29/2020, so one year ago. I was a returning player and didn't know about the legendary armory, otherwise I wish I had waited. 😞
As it stands right now, future Eternity crafters who create the weapon using the new token system will keep their Sunrise and Twilight to pull out of the armory at anytime, giving them 3 Legendary Greatswords. Anyone who have already crafted/bound Eternity will only keep Eternity (as they do not currently possess a Sunrise or Twilight), so 1 Legendary Greatsword. I feel like this takes away the unique 'sacrificial' element to making Eternity.
To put it into perspective, if you own Twilight and Sunrise right now and wait til armory comes out, you'll keep the 2 Legendary Greatswords, and use the Tokens for craft a account-bound Eternity.
If you have already bound Eternity, you just have Eternity, no Tokens, and presumably no account-bound Sunrise or Twilight. SEE FINAL EDIT.
I wish Anet had considered current Eternity owners alongside future ones when developing the armory. While not necessarily optimal solutions for everyone, Anet could consider giving bound-Eternity owners all three Greatswords in-armory on release day (Yes, even if it was bought from the TP), or removing Twilight/Sunrise from armory with the token deal.
TL;DR: If you plan to craft and bind Eternity, wait until after armory release to have 3 legendary Greatswords to use simultaneously. Don't be like me and every other current Eternity owner. Disclaimer: They may still change this prior to armory release, but as it stands now this is how Eternity functions.
EDIT: Theres a lot of discussion regarding why care about having three legendary greatswords in-armory. I and other collectors want these items in-armory without having to obtain them a second time. FINAL EDIT: ArenaNet repsonded, take a look! ALL current account-bound Eternity owners will receive in-armory Twilight and Sunrise as well.