During last year's Feast of Winter Veil, Lumpy dropped for my non-collector hunter rather than my collecting toon. I never learned that pet, and I still had the item sitting in my bags. Because account wide collections were still unconfirmed at the time, I held off using it in hopes that Blizzard would either guarantee the new feature or make pets Bind on Account.
Well, now that account wide collections seems like it's here to stay, I decided to test it out. I copied my hunter alt onto the beta and learned Lumpy. It was added into her personal collection as well as the Pet Journal.
After switching to my druid (main collector), I found that although Lumpy was not found in my personal spellbook companion tab, it was accessible through the Pet Journal. Yay! :)
I'm not sure if the spellbook companion tab will continue to exist once the Pet Journal is fully implemented, as it seems a bit redundant to me. Then again, it does serve as a useful tracking tool if you plan on keeping track of separate collections on different toons. We'll see what Blizzard decides to do.
Keeping both the new Pet Journal and companion tab in game does present a new issue though, which collection should you refer to as your "main" collection? The one exclusive to each character? Or the Pet Journal that acts like a stable for all your pets from every toon that you have? It's hard to say, and I guess it will be up to the individual to decide.
Anyway, I can finally breathe a little easier now that I know I'll be able to use Lumpy on my alt and then summon it on my main. I just have to remember that this is the beta still, so things could change. Hopefully for the better and not for the worst!