Dual log in PVP means two people shooting, or one person looting/scouting with the other shooting.
IMO this should mean unless you're out farming the taxi should be wearing nothing but a bluesuit to indicate such.
This would avoid having two PVP capable accounts with the one that wasn't shot shooting back, then claiming that you only used one account for PVP.
Having an alt help another account move around isn't really horrible OMG PVP Dual log. But I think its kinda borderline since its giving direct support though not in the encounter itself. Still nothing that should get someone banned.