Hi Jon,
We cannot compare tab and iView of portal to group and tile of FIORI launchpad directly. What you can do is,
Create a generic group which can hold both Tile A and Tile B. And separate groups with Tile A and another with Tile B. If a certain user requires both the tiles, assign the generic group to that user and not the other groups. Restrict the applications based on the user roles and authorizations.
Second option is create a catalog with tiles for group 1 and separate catalog for group 2 and assign accordingly.
Regards
Sabarinathan C