This isn't something that can just be answered. There are numerous ways to do this.
You can go low tech with IR emitters and retroflective tape (or retroflectors) with IR sensors.
A more high-tech solution would be to utilize ultra-wide band transceivers. With UWB you can pinpoint your rovers in 3 dimensional space around each other.
You could attach wires to two corners of the surface and mount two motorized spools on the robot so that the wires can be kept under tension. The distances to the two corners define the position of the robot uniquely. The design is very similar to this plotter bot: https://www.youtube.com/watch?v=T0jwdrgVBBc