If I understand correctly, Phil started writing his series first. That tutorial prompted several people to start working on their own hobby OS, including Steve's and another one here: http://www.randomhacks.net/bare-metal-rust/. I think Redox was already underway when Phil started writing, but I may be mixing up my timelines.
Phil has stated his series is basically for fun and learning. I imagine Steve's and Eric's are similar. You don't necessarily want to "combine effort" with someone when you are exploring ideas on your own.
However part of it's effort is directed into producing tutorials/book material for the educational etc use.
It's obviously up to 100% to them how and where are they going.
From the educational perspective it's _imo_ something worth considering at some point. E.g. combining this collective know-how into some sort of book. I know I would buy it.
We are all very friendly with each other, and hang out in our IRCs, and #rust-osdev. I expect the situation to be better for all of us, as we can cross-pollinate, and improve each other's work. Ecosystems can be better than single monolithic projects.
There are a LOT of decisions that can go into making an OS. The compare/contrast in a few years should be really interesting.
Redox was started independently by an OS hobbyist. It was linked to reddit.com/r/rust by a member of the community. It's since gone on to gain a moderate developer base within the Rust community.
http://intermezzos.github.io/book/ explains. We are sharing the "get into long mode" code, but after that, will diverge. Phil's tutorial is directly resposible for getting me back into OS dev.
Edit: There is also http://www.alexeyshmalko.com/2015/bkernel-a-rust-operating-s...
Phil has stated his series is basically for fun and learning. I imagine Steve's and Eric's are similar. You don't necessarily want to "combine effort" with someone when you are exploring ideas on your own.