There is currently no way to play coop on either Halo: PC, Halo: CE or Halo 2: Vista it'll prolly change with the upcoming release of the MCC on PC, tho I wonder what'ill happen to modding the MCC drops in, it'll prolly be weird for a while...
I'd love to see either Project Cartographer work on H2: MCC or H2: MCC features being brought to Halo 2: Vista with Project Cartographer (I'm not sure which one is the most realistic to expect)...