There is something you can try for free. I did it on my Bronica SQ when I had it. I made a panoramic mask and taped it to the film back in front of the film gate. Made another one for the VF screen.
I'm assuming the ETR works the same way as the SQ. You load a 120 film, advance to frame 1 then toggle the multi-exposure switch. Take the first picture. Then cock the shutter and manually advance the film via the winder on the film back. That's where it gets a bit iffy; you need to advance one half-frame, that is eyeball the advance when it is half-way between the 1 and 2 on the counter. For the third photo, the winder will stop correctly at 2. And so on...
For an ETR, the mask would be ~21x56mm. You would get 24 panos on a roll of 120. But you can remove and replace the mask via the dark slide to alternate between 645 and pano.
When I did it, the spacing was wonky. But it worked and it was free.