How do you use two lidars that are placed at a diagonal(scan area of 270 degrees)?

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This very much depends on the application. It is fairly easy to convert both LIDAR's data to a single pointcloud by first separately converting them, transforming them into a common frame and the combining them. This would largely work correctly for consumers that are only interested in the scan endpoints of both sensors, but raytracing a path from sensor origin to endpoints (as many grid mapping approaches do) would no be possible anymore.

See the laserscan_multi_merger in ira_laser_tools for an implementation.