Sharing 'capabilities' between installations

I can work with HTML, php and sql I only know enough of to know what may go wrong and use Google or something to seek a solution. But I’m willing to expand my horizons because code should serve our desires and WordPress is an excellent platform to work off.
This has been discussed before, but I’m a bit rusty on my America-English, I find these old discussions to be hard to follow.

tl;dr – I’m slow. But I do pay attention.

I’ve been looking for a way to keep 2 (for now) blogs and give my users the benefit of not having to register twice and be able to share certain information between them.
I’ve managed to tell my second installation to use ‘users’ and ‘usermeta” tables from the first installation with the help of this gentleman. This is where I ran into the crux of my problem: according to support staff and programmers on this forum the second installation can’t use the user ‘capabilities’ from the first installation.

Why might this be? If I had a plain-English cause-and-effect explanation of what’s missing I’d be certain of where to go next.

For reference I found this support threat, which I’m sad to say confuses the hell out of me, it’s all over the shop. But that didn’t stop me trying to implement the hacks prescribed by Wolthers. I don’t know what any of the code is saying so I don’t know how to alter it to my particular installations.
Upon implementing his hack in wp-config.php and /wp-includes/capabilities.php, I get SQL errers in config concerning the lines I changed. Upon removing those lines it reported lines that belonged in there. For the laugh I deleted those and it reported an error in capabilities.php. So I uploaded the backups and now I’m here.

Then there’s the issue of cross-domain cookies. But I’ll drink that ocean when I get to it.

Cheers