Of course I'm referring to side by side and not stacking or overlapping. 200 x 200 mm PEI and Kapton sheets are more readily available than 400 x 400 sheets. I'm wondering if anyone has tried this and if the edges of the sheets cause a problem.
I haven't tried such a thing, but a speculative answer covering the constraints and expected failure modes may suffice here.
Unless the sheets have squared (rather than rounded) corners, the corners will almost certainly be a problem. In particular you'd end up with a hole right at the middle of the bed.
If the sides aren't entirely square (perpendicular) with each other then you'll have large gaps at one end or another.
As Trish noted in a comment, it's also possible that you may hit problems due to mismatch in thickness between the different sheets, although I'd expect these are fairly well-controlled in the manufacturing process; otherwise I'd expect bed leveling issues even with just a single sheet. If you do hit this kind of issue, it may be possible to avoid it with shimming of some sort.
Assuming you can avoid all of the above potential problems, I would expect what you've proposed to work reasonably well, especially if you're ok with very minor surface defects at the seams.
Aligning build surfaces isn't the main issue with 1 mm thick surfaces when aligning four 200 x 200 mm surfaces to make one 400 x 400 surface. The main issue is slight bucking at the seams from thermal mismatch. Using this with PETG tends to tear up the build surface at the seams. The image shows aligned surfaces.