Is there a way to move multiple rigged characters simultaneously? Is it possible to select multiple root bones, in multiple armatures, and then translate them simultaneously?
Not without some custom script (custom modal operator etc.).

The workaround is to child_of the root bones with constraints to Empty and move the empty. Then Pose > Apply > Visual Transform to Pose and delete the constraints.

There is an addon that lets you setup child_ofs easily or to keyframe them even: Dynamic parent