You can launch the roscore on your laptop (or any ROS device that stays connected during the experiment. it could be one of the robots) and then launch your two robots with roslaunch commands. If they are on the same network and you have correctly specified the ROS_MASTER_URI you should have no issues.
Also, if you have multiple robots, make sure that you launch them in separate namespaces (e.g /robot1/,/robot2/ to avoid confusions and conflicts ...