Generally if both tanks start a fight with their tank stance on, agro is going to go to whomever does the most damage in short window after pulling, and thats not even a gear thing, just a class thing. For example, DRK has the highest initial burst in the game followed by GNB and WAR etc etc.

Generally? If you're OT, just turn your tank stance on like 3-5 GCD's in to the fight, by that point the MT will have enough of a lead it shouldn't be a problem, and it'll mean you'll always be #2 on agro if something goes wrong or if a mechanic calls for it. Some fights call for specific agro management but you seem to have a decent enough grasp of it to be able to adapt if needed, but its a good general rule to follow anyway.