Just to clarify: you’d remix material from both series. One team will select the source tracks from one series (one track per game, three games) and the other team will do the same for the other series, and then the sources will be paired up. Each team will produce three mashup mixes, one for each of the three pairs of sources.
In the previous two GSMs, teams have taken different approaches to getting tracks put together with regard to whom does what. You might be on a team with three or four performers and a couple of mixers, maybe one team member is good at mastering, and three arrangers; or your team might be five arrangers and someone who plays guitar or something. As long as your team gets their three mashups done by deadline, it doesn’t matter how you do it. :)