It seems the general consensus for lead-free solder reflow is approx 220 C. I would think that anything between 220 C and 250 C would be ok, but don't quote me on that one.
As far as Individual temp settings, my guess would be that they should be the same temperature. The Xbox motherboard isn't extraordinarily thick or complicated, so applying heat to just one side will almost have the opposite side at the same temperature. Applying equal heating to both sides I imagine would help reduce a little stress from heating.
Duration is a little trickier question. You want to heat the motherboard just long enough for the solder to reflow and then immediately reduce heat to reduce the chance of damaging components. If you have an infrared temperature gun, that would be a perfect tool for this application. You want to run the reflow station until you read ~220 on the surface of w/e you are heating (measure with the station OFF). Then you probably want to run an additional 30 seconds of heat to make sure the heat has fully penetrated the board and components.
Hope this helps, let us know how it goes!