I would like to confirm that running two versions of darktable in the same computer only requires to start the second darktable version as follows, in order to ensure data integrity (database and thumbnails):
./darktable --configdir /home/user/.config/darktable2
Also, I’d like to know if there’s a problem if both versions use the same cachedir.
I’m currently running four different versions! I start them all with different config and cache directories. Don’t know whether I need to but I have plenty of disk space.
Thanks.
The point is that I already started the second version pointing to two different databases, but sharing the same cache.
I wonder if that will cause loss of integrity between the first database and thumbnails, or if I can proceed safely with that. For example, will the thumbnail numbering be messed up, since thumbnails relate to two different databases?
I would have thought you could just delete your existing cache and start again with two fresh ones.
I see.
I was refraining going that way because I recently moved all images to a nas and created all thumbnails locally in my computer. That took a looong time 
But maybe I don’t need to have them all created at once in my computer.
EDIT: I think I’ll do the following. Since I edited a very small number of images with the second darktable version, I will stop using the original cache before it’s too late and start using a brand new one. If some thumbnail inconsistencies arise in the first darktable version due to the images I edited with the second version, it will be only a few and they probably will be recreated as soon as I enter darkroom.
cache location is bashed on sha1 sum of library locations (just so cache from 1 library doesn’t contaminate the other).
Therefore - you’re safe.