Has anyone else heard of processing in the film holders?

An acquaintance of mine was telling me how he has processed sheet film in the film holders.

He has a rectangular light tight tank with a sealable top and has adapted dividers to hold the film holders vertical. In the dark he pulls the slides of 4 dual side holders, puts them in the tank and pours in the developer. Put on the top and the lights can go on. He follows the same process he used for roll film - tank inversion every 30 seconds - lights go off and pours out the developer and does the same for the stop bath and first fixer as he uses a two-step fixing procedure. He removes the film and used a tray process for the second fixer and then the wash.

I was concerned about contamination and staining with chemicals trapped behind the film in the holder and maybe that is one reason he uses a second tray for fixing. He claims he hasn’t had a problem, though he has only done B&W whereas I often do color and contamination could be a concern. He has always done it this way as he was shown by the person who sold him his setup years ago.