If those are your only choices go with the Odin pro 800 the coolmax 850 is not a good power supply nor is it reliable.If you look the Odin up it has great reviews.The Gigabyte Odin Pro 800w PSU with 4 +12v rails combined rating of 62a
GeForce GTX 660 Ti - On your average system the card requires you to have a 450~500 Watt power supply unit.
GeForce GTX 660 Ti SLI - On your average system the cards require you to have a 700 Watt power supply unit as minimum.
For a system using two GeForce GTX 660 Ti graphics cards in 2-way SLI mode NVIDIA specifies a minimum of a 650 Watt or greater power supply that has a combined +12 Volt continuous current rating of 38 Amps or greater and that has at least four 6-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the most important factor.
Overclocking of the CPU and/or GPU(s) will require an additional increase to the combined +12 Volt continuous current ratings, recommended above, to meet the increase in power required for the overclock. The additional amount required will depend on the magnitude of the overclock you are trying to achieve.
The GIGABYTE ODIN Pro 800W (GE-M800A-D1), with its combined +12 Volt continuous current rating of 62 Amps and with four (6+2)-pin PCI Express supplementary power connectors, is way more than sufficient to power your system configuration with two GeForce GTX 660 Ti in 2-way SLI mode. There is ample spare capacity to handle any overclocking of the CPU and GPUs you may want to attempt.
If you're going to be plugging in the GIGABYTE ODIN Pro 800W (GE-M800A-D1) to a UPS (i.e. Uninterruptible Power Supply) it will most likely have to be a UPS that outputs a pure sine wave when operating on battery backup mode.