Community based ofc. A bit like TWC (TWC had Native and NW) but with a round/point systems that actually makes sense and isn't skewered towards one side. In TWC you played your opponents across both modules and whoever had the most points from "sets " won the match. Of course sets didn't make any sense on NW.
As a team you submit one lineup, that same lineup plays all games. Then you'd do a ft15 on NW and a ft16 on CS. You would also need a third game in case of a tie, so you could do a format on Rocket League that is comparable, maybe FT3 in-game match wins or something. (I'm not that familiar with how anything competitive in RL works). You then assign 1 point to each victory and whoever gets 2 points wins. You'd have to play all 3 games as a team. So a team has to decide if they want to be CS and RL focussed, or CS and NW, or RL and NW. etc etc.
