You should put the EVGA in first because the second card will have to lower its clock speed to match it (Gigabyte has a higher clock). Doing it the other way around would most likely require some type of boost or overclock.
Yes you can mix the same card from different manufacturers, however it is not usually recommended due to different clock speeds. Make sure to use the EVGA as the main card (1st pcie slot) as the second is going to have to scale to the first.