multiple Aperture Libraries and Photos
It seems Photos can deal with only one Aperture Library, when I import the 2nd Aperture Library overwrites the 1st one. Any ideias?
Mac mini, macOS 10.14
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Photos is not overwriting the first Photos Library. It will migrate each library as a new Photos Library. The new Photos library will be created in the same folder as your Aperture library and have the same name, but the filename extension will be .photoslibrary instead of .aplibrary. The original Aperture Library will have the filename extension changed to .migratedaplibrary.
You can switch between Photos Libraries by double clicking them to open them in Photos.
And to open the original Aperture Library again in Aperture change the file bane axtensionback to .aplibrary .
There may be one complication. If your Aperture libraries are on an external volume that has an incompatible file system format or has been used for Time Machine backups, Photos will try to create the migrated library on the system volume instead. You should make sure, that you migrate your Aperture libraries on a suitable drive for Photos. The file system format should be MacOS Extended(Journaled) or APFS, not case-sensitive, and the volume must not be a Time Machine volume and needs to be locally mounted, not a remote volume.
Photos does not have the ability to merge photo libraries. You cannot import a photo library like we can in Aperture. You also cannot export partial libraries. If you want to merge your libraries into one library, do it, while you are still running Aperture and use Aperture to merge your libraries before the migration to Photos. The only lossless way to merge Photos Libraries is uploading the libraries to iCloud, one after the other, and they will be merged in iCloud Photos Library. Other shortcomings of Photos are the lack of hierarchical keywords and a poor support for referenced files. If you want to relocate referenced original files do it while still using Aperture, or consolidate your referenced files. A referenced library in Photos will sooner or later become a problem, if you need to relocate or reconnect the originals.