You can't crossfire Nvidia cards. Crossfire is for AMD cards. In Nvidia, you SLI (not crossfire) the cards. SLI doesn't support different GPUs, it has to be the same chip. So, no, you can't pair the GTX 1060 and 1080 with each other.
SLI'ing two weaker GPUs is not recommended over getting a single more powerful GPU. But if you really need to SLI two cards, you need a motherboard that has at least 2x PCIe x16 slots that run in x8/x8 speeds.