SLI is a way of using two nvidia cards connected together using SLI bridges to power one monitor so you can get more performance. If you want to use two AMD cards linked together to drive one monitor then you need a crossfire compatible mobo. As AMD don’t charge as much to license the tech as nvidia do alot more mobos support crossfire than SLI so I can say with a very high degree of certainty that if you want to use 2 AMD cards you dont’ need an SLI compatible motherbaord.
Putting more graphics cards into a compuuter to drive more monitiors has been around since long before nvidia even thought the idea up Aslong as there are the available slots (and the PSU can power them) then you can put as many graphics cards in your pc as you like.