I've played since ARR and I'm telling you, that won't happen. Nobody cares who the main tank is in queue-for content. I have the 10,000 duty achievement - probably twice over if it would show, so I have more than enough experience to say this.

Now let's say a PF already has an off-tank. Whether they will take you, will depend on a few things. First, whether there is a % bonus for having a MT and OT, or whether the bonus is just for "a tank". Precedent says it won't distinguish between MT and OT for the damage bonus. The second thing is whether the fight specifically depends on something only an MT can do. If none of these things are the case, then 99.99% of PFs won't care if there are two "off tanks", and this is the most likely scenario by the way. Off tanks have to be able to tank since they need to work in 4-person content like dungeons and criterion.