You can switch out of SLI mode in the drivers. Jumpers/switch is required to change between 1x16x PCIe slot and 2x8x PCIe slot, not to turn on/off SLI and dual cards.
There should now be support for switching between using 2 cards (for 4 monitor support) and SLI mode in the drivers, and I think it's switchable without a reboot.