Hi Alexmark,
To address your original post, I don’t know the down side of stacking the toroidal transformers. I would have thought as you have, that the mutual inductance would be a problem.
Ideally, mutual inductance is minimized by, first distance, but I’m assuming this is not an option. If they must be mounted in proximity to each other, then they should be mounted at right angles. One can be placed flat against the front panel and the other would then be placed as it is in your photo. This would set them at right angles with mutual inductance minimized.
This said, I’ve seen plenty of projects on this site that ignore this property completely, stacking the transformers on the same center, and they don’t seen to be having any problems. At least none that have been talked about to my knowledge.
I think I would take the gamble and stack the transformers on center to save the space. Maybe one of the guys that have done this will chime in and let us know how it turned out, but suspect it’s not a problem.
Just my thoughts,
Rodd Yamas***a