1. The MB has to be licensed by Nvidia to do SLI. The PC's motherboard should ship from the factory with an SLI bridge included.
2. Two PCIe x16 slots capable of running in tandem at (x8 mode).
3. Adequate power supply with supplemental PCIe (six/eight pin) power connectors.
Some Omens are SLI ready, some are not SLI ready.
Most motherboards will have two PCIe x16 slots close to the processor. You want to use these two x16 slots. The SLI bridge will fit the gap between the two x16 slots so you can use the bridge to connect the graphics cards.