I'm going to compile my discoveries into one thread so that it's collected. The Japanese and English trailers are different from each other so there's actually two videos to pull info from.
Second of all, THE ITEM SYSTEM. Underneath your healthbar is a diamond shape where you get items. I THINK these items are based on your partner choice. Earlier, Mai got this tornado item, then Mai uses it after a tag.
Leona here does the first two hits of her auto combo and then CANCELS INTO RUN AND THEN DOES HER RUN ATTACK, and then tag cancels the run attack. Tagging has super freeze during hit?
Here's somewhat proof that this isn't a one button game. Kula does st.A into st.D. The fun stuff though, st.D ground bounces and she JUMP CANCELS ST.D INTO JUMP D. Athena juggles after the DP with jump C into Phoenix Arrow. Normals juggle.
I was probably way overthinking the "tag meter" thing. It's more likely that your two characters have their own special meter. So your two characters share health, but not meter, and you tag them out to recover meter.