A few ideas:
More mirrors to make the path lengths match
The first is to use more mirrors such that the path lengths are much more similar, while hopefully not increasing the total by very much. Here's a sketch based on yours. The up and down parts of the extra path in the top beam are each half the lenth of the extra path distance in the bottom beam.
It is of course possible that you don't have space for this
The second idea involves using a diverging lens in front of the camera lens for the mirror path only. In microscopy this is referred to as a Barlow lens (note that the term is used slightly differently when applied to telescopes).
This would need to be a weak negative lens, ideally an achromat but a meniscus lens might be enough. It would need to be mounted square to the optic axis quite near the existing lens and aligned in three dimensions, with a reasonable amount of travel in the focus direction. You may need to have the lens cut down to clear the path to the other image.
Commercial split field filter
A "split field filter" (link to a Cokin product but I suggest a google search) would seem like a good idea, but I can only see +1 to +3, which are probably too strong. This is a little like a manufactured version of what I describe, but without all the adjustment. Also the filtered bit would have to go in the shorter path. The lack of translation along the optic axis means that tyou have little control over the distance ratio that's in focus.
A tilt-shift (or just tilt) lens is designed to do things like this. By introducing a slight angle between the lens and the camera the magnification and focus plane change across the image. The effect on the magnification may be tolerably small or calibrated away. This adjustment is normally built into the lens. I've seen various projects to add tilt to an existing lens, they all at as extension tubes as well, reducing the maximum focal distance.
These lenses are expensive, and because the effect is continuous you may struggle to get enough depth of focus within a given half-image