Your ROSbots are publishing tf frames with prefixes. We are considering base_link frames, these are:
first/base_link - for one of the ROSbots
second/base_link - for the second ROSbot
Move base is configured to search for frames
base_link, it is without prefixes.
map frame is published without any prefix, this one will not cause any problem.
Issue is caused with
base_link due to fact that, ROSbots are publishing with prefixes, while
move_base wants it without prefix.
Warning is thrown by costmap component, the frame definition will be in one of the files:
Set the searched 'base_link` frame with appropriate prefix.