Ah, does your dashcam see backwards from the same housing (via a more telephoto lens) rather than having a completely separate camera stuck to the rear window? Is that the normal way rear-view dash cameras work? Presumably the view is blocked if you have a third passenger on the rear seat in the middle.
I'd never thought of mounting a dashcam directly under/behind the mirror fixing, because so few modern cars allow that. Our Honda CRV has a huge plastic V-shaped housing which contains the mirror mount, the rain-sensor for the wipers and a camera for dipping/raising the headlight beam automatically if you enable that feature. When we first got a dashcam, we mounted it slightly on the driver's side of that V-shape, and noticed that the headlights no longer reliably dipped and went back to high beam. Then I realised that the dashcam was obscuring the headlight camera so I moved it to the corresponding position on the passenger's side. On most cars, you also have to take into account which parts of the windscreen are cleared by the wipers: the last conspicuous places. near the edge of the screen, are usually outside this so the camera just sees raindrops and dirt.
I have a tablet holder which has two suction mounts, but they are quite close and probably would not span such a feature. Bought it online a number of years ago and I don't recall the supplier. I guess the other option would be to canibalise two standard units.
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