I think there are two relevant rules:
Some Borg Command Cards will direct you to move multiple Cubes. In this case, move all the Cubes, whether or not they have Activated this turn, then complete the actions of the Cube which drew the card. Once all the Cubes have completed their action sequence, the Borg turn is over.
Sectors may not contain more than one Cube. If a Cube’s movement would have it end in the same Sector as another Cube, stop in the Sector before the Cube already occupying the Sector. After the moving Cube’s actions are complete, activate the stationary Cube, even if it has already gone this turn.
If you are playing RAW, I suppose any cubes that would end in the same sector will activate cubes already there again.