Is it possible to use DJI FPV Remote Controller (https://m.dji.com/product/dji-fpv-remote-controller) with additional radio in buddy box mode / trainer mode / coach mode?

It is required by law in some places when flying FPV to have a spotter that can take control any time of the aircraft if something goes wrong. Usual solution was to set up 2 OpenTX radios in trainer mode, but DJI FPV seems to provide no such option.

If it's not possible to set up buddy box on the DJI FPV Remote Controller side, would it be possible to set up 2 receivers - e.g. FrSky receiver with SBUS in connected to DJI Air Unit output, so that spotter with Taranis could override the DJI controls?

I wrote to Joshua Bardwell and this is the response I got:
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I don't believe the DJI system has buddybox support. In the United States, this requirement is sometimes satisfied by having the student hand off the controls to the instructor. If you had an RXSR receiver on the quad, perhaps you could feed the SBUS output of the Air Unit into the SBUS IN wire on the RXSR. This is intended for redundant receivers. But the problem is, you would need to totally power down the DJI system before the FrSky controller would gain control. At that point, you might as well just hand off the receiver.
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Therefore, buddy box is probably impossible with current DJI FPV, and the workaround with RXSR won't work well because it would require introducing complexity of powering down the Air unit.