gtx 1080 ti sli

So,basically I have a 1080 ti and a 1080p 75hz monitor.I am thinking of getting a 2k 144hz monitor and I wanna upgrade my gpu.I don’t want to get a 2080 ti because it is too expensive and not much of a difference so I thought of getting another used 1080 ti for about 300-400€.I heard that sli is...