Hm, that's odd. I just did a quick test--copied a few images into a folder that were taken with three different cameras over the course of the past several years. I changed the sort order in Digikam to date ascending, then did a rename (sequential number) and changed the sort order to filename. It worked as expected. This was with less than ten images though; maybe if you're trying it with a larger set of images that would trigger some bug. A few things that might be worth checking:
1. Are the images in the correct order before the rename? This would help narrow down if the time on one camera is incorrect, as suggested above. If not, I would suspect date/time settings that aren't synced between the two cameras.
2. Has any post processing been done to the files? If so, perhaps the image editing software you're using isn't preserving the exif date/time stamp.
3. In the rename window, are the files in the correct order?