You can get 50/50 of a team and 25/50 of another with some extra player leeway to include your favorite players that are not part of either team. You can also utilize the specialist tab to put players that you want to use that are not part of the theme teams. - Note that the players put in the specialist tab that aren't in your offensive/defensive lineup don't count toward chems however, they still get chem boosts.
The trick is to utilize the players that get all team chems on the card like Emmanuel Sanders (all AFC + extra chems for teams he played for) and yes he gets the boost for both teams - 96 spd + 2 from 50/50 and + 1 from 25/50 = 99 speed! Other notable players like this are: Rodger Saffold III (AFC), Ndamukong Suh (NFC), and Jordan Whitehead (NFC) of course they also get an additional chem slot for any teams they've played for.
Make sure you plan your team out before you build it some teams have overlap and you might be short depending on the combination. Use the https://www.mut.gg/forums/forum/mut-lineup-helpfeedback-15/topic/list-mut-players-with-x2-and-x3-team-chemistry-2837/ to find the players that get multiple chems for the teams you want to use.