Suggested By OriginalA
Clover, Alex, and Sam (and parents) are taken away from their universe and transplanted into the KPverse (specifically replacing the Stoppable and Possible house holds as well as a random nearby neighbor). Meanwhile Kim and Ron (and family) are taken from their universe and transplanted into the TSverse (same deal; they take the place of two of the three Spies’s family homes).
Their support crews (Jerry and his WOOHP organisation on the Spies’s side and Wade on the KP side) are doubled so that all WOOPH personel and Wade (plus his family since he lives at home) exist in both the KP and TS universes. This is so that A) Kim and Ron still have their mission control through Wade, B) the Spies still have their mission control Jerry, and most imporantanly C) the TSverse doesn’t turn into a giant mess due to WOOHP suddenly disappearing (there are several episodes in TS where the villians require WOOHP to exist as well).
Both sides start at Season 1 Episode 1 in the other universe and both sides start as their S1E1 incarnations. That also means that they keep their personal interests (e.g. Kim would still be a cheerleader even though she is now in a TSverse and the Spies were not cheerleaders).
Their mission is to finish the plot line of the other show up through High School Graduation.