As we all know, 
and 
work together. But what else increases damage done by ? Does the rest of your armor, including Guardian/Noble buff? Does Knight synergy get added? If I stack 2 , do they both do full damage for each other's mitigated damage? (For example, if one mitigates 100 damage, do each of them recognize that 200 damage is mitigated and each do 200 damage?)
Yes, they stack. Mitigated dmg = attacker's dmg - the dmg you actually took for example a draven crit for 400dmg, you have 200 armor, so you are taking 100/(100+200)=33.33% dmg, approx 120dmg mitigated dmg(dmg reflected by thornmail) = 400-120 = 280
I believe they stack, given the lack of a 'unique' passive on them. I don't know if armor diminishing returns are calculated in the same way in TFT as on SR, but if your given amount of armor produced 50% mitigation, then yeah you should reflect the entire damage of the auto attack back at them if wearing 2 TM's. And yes, it applies to your total armor, so anything that increases armor also increases the damage reflected back.
Shielded damage, Immune to critical hits, Magic damage reduction, Blocked Knight damage.
