Another interesting trick is when you want to directly compare 1.25 eyepieces. You just install each right next to each other!
...assuming you have a perfect beamsplitter.
... and they are parafocal
... and your eyes are equal
Comparing different eyepieces in binoviewers at he same time would only work is powers were within 10% or both eyes would not merge images. Assuming your eyes can accept that much difference, and the in/out focus is within limits to look through with both eyes. Might be a good way to show different apparent fields of view with at same power (but most individuals probably wouldn't have that type of duplications).