Can two users really share a photo library?

You can share the same library between users, provided only one person uses it at a time, and you do not try to do this on network volumes. But if you keep the library on a separate disk partition or external drive with the proper formatting (MacOS X Extended) permission settings, it is possible to share the library, even if one user is using iPhoto and one user Aperture, see this support article:

iPhoto: Sharing libraries among multiple users

Also, for more information on how to use Aperture and iPhoto on the same library see:

Aperture 3.3: Using a unified photo library with iPhoto and Aperture

Regards

Léonie

That refers to the *applications* sharing the library (meaning, you can open the same library in iPhoto or in Aperture). It doesn't refer to *users* sharing the library. Users can't have the same library open at the same time, whether they're both in Aperture or one is in iPhoto and the other in Aperture.

It's not clear to me. This part of the text cited by the OP ...

The unified library is great when there are two people who use the same Mac for both iPhoto and Aperture.

... specifies two users. I think we all agree that two (or three hundred) users can use the same Library on the same machine as long as they all use the same User Account. What I think needs to be clarified, is if two users with their own User Accounts can use the same Library on the same machine without any special set-up. I think the use Apple has in mind -- quite common, IME -- is two members of a family who want to each have access to their family photos. (Not at the same time -- they are on one machine -- which the OP specifies.) We all take it as a given that only one user can be the active user on any machine at any time.

Maybe it means two users with the same user account - one that prefers to use iPhoto and the other that prefers to use Aperture. That's certainly possible.

But "sharing" the library across user accounts, while possible, takes some work because of the way permissions in OS X work. The first article that leonie linked to probably addresses that.

The first article that leonie linked to probably addresses that.

Exactly - that knowledge base article (iPhoto: Sharing libraries among multiple users) describes how to use a disk (partition) with "Ignore ownership" flag enabled to share a library from different user accounts.

Per JWaterworth's post: I still don't know whether two of us can access a shared library from our separate accounts (logins). A genius at my local Apple store told me I could merge our separate iPhoto libraries, then move the combined library to the shared folder on the hard drive. Has anyone done so successfully? Does that approach require special setting of permissions?

. A genius at my local Apple store told me I could merge our separate iPhoto libraries, then move the combined library to the shared folder on the hard drive.

That used to be possible with older iPhoto versions, but a shared folder does no longer work. Read Apple's Support article I linked to:

iPhoto: Sharing libraries among multiple users

Important: If two or more users attempt to use the same iPhoto library and the library is not on a volume where permissions are ignored, unexpected results may occur. This includes the inability to see or edit certain photos, or the inability to open the iPhoto library.

You need to move the library to a separate partion or an external drive, where you can set the "Ignore ownership" flag.

Thanks, Leonie. I'll have to study up on partitions and flags, but it sounds like something I could do.--- Dave