I've been running around for most of the game with both ED-E and Veronica, and multiple times during loading screens I've seen a tip suggesting I keep two followers with me for twice the perks.. I picked up Cass, and now apparently she's a solo only companion so.. WTF!?

Cass, Veronica, Boone, Lily, Arcade and Raul are mutually exclusive
Then you can add either ED-E or Rex to that

You can't have ED-E with Cass? Is that what you're saying?

Keep in mind (if you haven't) that it is one non-human (ED-E, Rex) companion and one "human" (Veronica, Lily, Cass, Raul, etc) companion that you can have. Unless Cass is some sort of special case (I dunno, I haven't hired her yet).

You can have 1 cybernetic follower (ED-E and Rex) and 1 flesh and bone follower

from what i have heard i guess i could be wrong i heard you can have a non-human follower and a human and in new vegas a ghoul can be a non-human follower...

When I first tried to get Cass in my party I sent ED-E off first, he's just a mobile gun with a massive body cavity for hauling loot after all, and he was empty at the time, when I still couldn't get Cass I sent Veronica home too, hence my confusion.. Now I've lost ED-E too, He's not at the Lucky 38 or Primm

As I and other people said, only one companion of flesh plus one of metal at the time. And ED-E question got answered in your thread about it.