"Why and how are Marcus and Dom in Lost Planet 2" is a question we can't really answer. They just are, and we had a chance to take them for a rumble in the jungle during Microsoft's X10 event in San Francisco.
Content-wise, Capcom was showing off a couple of new areas from the campaign ("we don't differentiate between single- or multi-player for campaign; you can either go alone, go with bots, or go with friends," notes a Capcom employee): specifically, areas 1-2 and 1-3. Moment-to-moment gameplay-wise, Lost Planet 2 plays and feels remarkably similar to the original game. Run around in third-person; shoot some grody-looking akrid (giant insectoid aliens) in their bright glowing orange spots; scoop up the orange goopy energy that akrid drop when they die; spam the B-button to activate data posts; climb into Vital Suits and smack akrid around. The controls feel the same in general -- the aiming does feel a bit tighter and "smoother," where it feels like a Western-made third-person shooter instead of one from Japan.