Tested in that way:
Opened second WoW window and typed in my password
Switched to first window and oomed my priest
Switched to second window and hit enter
Logged in my priest and did not had 100% mana
Since for me the forced disconnect of the first window due to logging in a second time did not work, I don't think it works when really disconnecting... I had about 60% mana when I got in. I think it was the OOFSR mana reg which made your mana bar become full.