piefed.ca hasn't had any outages since this morning, what were you seeing?
You only need to sign up for one instance, they all see each others content. Think of it more like email - you wouldn't sign up for both gmail.com and hotmail.com
For example these two both show the same community which is hosted over at lemmy.world:
https://lemmy.ca/c/technology@lemmy.world vs https://piefed.ca/c/technology@lemmy.world
Lemmy.ca and piefed.ca are both run by us (fedecan) and are just different software platforms to interact with the fediverse. Lemmy was the first big popular one and is written in Rust, but Piefed came along more recently and is written in Python (hence the Pie in it's name). Piefed has been quicker about adding features, and many people have issues with the politics of the Lemmy developers. Ultimately they both let you interact with all the same communities and posts.
Otter put a nice doc together at https://fedecan.ca/en/guide/get-started which has some pictures that might help you understand it all. It can be a little confusing at first!