Are you thinking of doing this on different domains, sub-domains or folders of the same domain? All three will work in version 1.6 (not quite finished yet), but the third option will work with version 0.9.1. You need to use stub files.

alrite i have my blog installed on:
http://blog.balupton.nghosting.org
(which is /blog)

And the Offline/Online urls are as follows:
http://dev.balupton.nghosting.org/online/
(/dev/online)
http://dev.balupton.nghosting.org/offline/
(/dev/offline)

But in the configuration for the baseurl in /blog i use:
http://blog.balupton.nghosting.org

So i would like it so
http://dev.balupton.nghosting.org/online/
Is actually
http://blog.balupton.nghosting.org/?blog=5

So
http://dev.balupton.nghosting.org/online/?author=1
Is actually
http://blog.balupton.nghosting.org/?blog=5&author=1

So can i do this?

What version of b2evolution are you using?

1.6

Yes, you can do this. It may not all work like it's supposed to, but most of it will. Create stub files for each blog and put them whereever you want them to appear in the subdomain folders. Make sure you adjust the path back to the evocore folder at the bottom of the stub file. Then edit the access parameters in the backoffice to use an absolute url.

Hmm but i don't see how stub files will work as the blog is installed to:
root/blog
And i want to access it from
root/dev/online

How about making a index.php file that will include root/blog/index.php
And then configure the Blog Folder URL to:
http://dev.balupton.nghosting.org/online/

Should work shouldnt it?
So does the "Blog Folder URL" thing work by, getting the current url, and see which blog that matches up to, and the blog it will display is the one that matches up?

Yes, a stub file will work. I've done it. Copy /root/blogs/a_stub.php to /root/dev/online/index.php and find this line in the bottom of the file:

require dirname(__FILE__).'/evocore/_blog_main.inc.php';

and change it to

require dirname(__FILE__).'/../../blogs/evocore/_blog_main.inc.php';

Alrite im new to the whole stub file thing.

But wouldnt what i said above work?
where i make dev/online/index.php which includes ../../blog/index.php ?
And configure the Blog Folder URL to:
http://dev.balupton.nghosting.org/online/

Should work shouldnt it, or do i need stubs?

I don't know if your way would work. Try it if you want. If it doesn't you can go with my suggestion.