They stack, at the same tic rate, assuming your HP% is above the amount to trigger the latent on the Parade Gorget.
That's why Ares's Cuirass + Parade Gorget + Auto-Refresh is so nice, 3mp/tick naturally without outside assistance.
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Main trait and sub traits, though, do not stack. That is to say, if you were to go PLD/SMN for some silly reason, you wouldn't be getting 2/tic just from your traits alone. Same thing would go for something like BLM/RDM; BLM main already gets all the Magic Attack Bonuses so /RDM would not add any more.