critical hits do not stack, ever, under any circumstance, the chances also do not stack or have any priority, a critical hit works much like an orb effect, such as life stealing. when you crit as a PR, and if you have multi-shot, you will notice your multi shot only goes off when u dont crit, this is because the crit over rides the multi shot ability, and is singularly the only action taking place. The crits themselves work in a way that, obviously the one with the highest % chance of occuring will occur most often, and the rest of your crits add minor percentages to crit for thier own certain amount. SO, each crit works individually, and you will never get a *double crit* if i can call it that... you will always do one of the amounts specified of ure crits, they wont add up.
on the priority note, there is no priority, its entirely based on percentage, highest percentage occurs most often, next highest occurs proportionately lower determined by how many crit giving items you have, and thier respective chances to crit.
Edit:
some examples of how the percentages work (kinda)
referenced from dota
1 Iron Buckler: 60% chance to block 20 damage ; 0.6 x 20 = 12 average damage block from each attack
2 Iron Bucklers: 1 - (1-0.6)(1-0.6) = 1 - (0.4)(0.4) = 1 - 0.16 = 0.84 = 84% chance to block 20 damage; 0.84 x 20 = 16.8 average damage block from each attack
referencing to pom foc
1 legendary satans ace: 51% chance to do 25x damage; .51 x 25(lets say 100 damage for maths sake) = .51 x 2500 = average 1250 damage on each attack
2 legendary satans ace and a legions sword: 1 - (1- .51)(1- .50) = 1 - (.49)(.50) = 1 - .249 = 75.1% chance to do either 2500 damage, or 1200 damage, split with 51% of 75.1 being for 2500, and 49% being for 1200.
IE if you have both of those, your chance to do a 2500 damage crit would become .51 x .75 = 38%~ and your chance to do legions swords 1200 damage would be the other 37%~
if you dont understand where the decimals come in and how/why percentages and decimals seemed to interchange randomly, just pm me and ill add an edit or something.