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The easiest way involves another camera.

First calibrate the camera intrinsics for all three really well.

Then get two calibration targets (chessboards) and place one in the FOV of each of the opposing cameras. Then use the third to take a video that contains both targets in the frame at the same time.

So to find the transform from Camera A to Camera B (the opposing cameras), you need to go Camera A -> Chessboard A -> Camera Ext -> Chessboard B -> Camera B

There's a lot of room for error there, so you'll need several views from the external camera per positioning, and several positionings of the chessboards to get good results.