If this happens then the best thing you can do is let the OT have aggro. A fight between tanks to spam things like provoke or skills that are meant to take Aggro from another means that if a mechanic comes up you won't be able to handle it properly. It sucks but the best way I've found to handle this is just let them tank until they die, then take it back and hold onto it. If they're dumb enough after that to try to grab it then you have a problem because at that point they have rez sickness so they're taking more damage and doing less. That is not your fault, and you cannot be held accountable if the group wipes because you at that point did everything you could to stop it. If the OT is receptive to a discussion then have a discussion with them about who should be MT. As a general rule, I default to the person with more health taking the lead because that's usually a sign that they have better gear.