Frame conventions in multi-robot simulation with ROS Navigation stack

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In any setup related to multi-robot simulation, where one wants to set up ROS Navigation stack on every robot, what should be the frame convention followed? I know in the case of a single robot, it is map->odom->base_footprint->base_link.

How would it be in the case of multiple robots? Should I have something like (assume I have 2 robots named alpha and beta) :
map->alpha/map->alpha/odom->alpha/base_footprint->alpha/base_link
OR
map->alpha/odom->alpha/base_footprint->alpha/base_link ?

Basically should both the robot shave their own map frame or should they have a global map frame? If my question seems redundant I request you to please explain why it is so.

TIA