Yaya
When you import a PSD file, images in the Images path with the same name are overwritten.
Are you using the same Images path for both PSDs? If yes, and your PSDs have colliding layer/attachment names, that’s the problem.
You should use two different image paths for two different projects.
Alternatively, you can wrap all your PSD layers in a group with the [folder] folder-name tag, where folder-name is different for each PSD. This way, even if the images folder is the same, the images will be imported into their own folder, preventing colliding layer/attachment names.
You can read more about the PSD tags here.
