@merlinuwe Hi!
An import to Bitwarden doesn’t “overwrite” your vault. You could do one import, and after that the other import.
Another method would be, to merge two exports to one CVS file before importing - but then you may have to condition the “complete” CSV, as both exports probably don’t have the same structure.
Oh, please don’t ask that (see this feature request: Duplicate removal tool/report (including merge)). Short answer: You will get double entries and have to manually sort that out.
You can delete the corresponding vault items afterwards. I guess a tip here is: create a folder before the import, then import to the folder. If anything goes wrong with the import, you only need to delete all items in the corresponding folder. (if you have two sources, you can import to two distinct folders then - and if everything works, you can reassign the items to other folders)