Most 285s work with other 285s. The only issue you may have is the next one you get if its faster clock will default to the slower cards speed or vice versa.
I have a BFG and an EVGA in mine. I did have 2 BFG but had a slight accident with one right before BFG went out of business and would no longer support their warranty.
Yes , as long as it is a 285 gpu you can sli two different manufacturers cards. Just make sure they have the same amount of ram. Also use Evga's precision software to link the two cards to the same clock speed.