how to share Quicken for Mac on 2 different desktops

You have multiple possibilities:

1) You can use the Quicken mobile app or web interface. This allows entry of transactions, but an overall limited subset of full Quicken capabilities. (I'll add the following caveat: while many people use this piece of the Quicken service, the syncing of data from the Quicken cloud to and from your desktop Quicken seems to generate a larger number of problems for users than any other aspect of Quicken. For that reason, I don't use it, and many of the other longtime users here don't.)

2) If the other person can work while you're not at your computer, you can set up screen sharing to allow the other person to access Quicken that's resident on your computer.

3) Pass your file back and forth via a cloud storage service such as iCloud or Dropbox, or even via email. The crucial key is that you either create a backup from within Quicken (a file ending with .quickenbackup) or quit Quicken and in the Finder do File > Compress (a file ending with .zip). Either of these types of files are safe to move between computers. If you use a cloud service as your repository, then whenever either of you wants to use Quicken, you'd copy the backed up file to your computer, use it locally, re-compress it, and copy it back to the cloud service. That way, you're always assured of using the latest version. (You'd need a system between you and the other person to make sure you don't both copy the file to your local computers and work on the files simultaneously. The system could be as simple as deleting the file from the cloud location, or renaming it, while you have it locally.) This may sound like a lot of steps, but it actually only adds a handful of seconds each time you start and stop using Quicken, once you get used to it. This is the approach I would use if I need to share the time on Quicken with someone else.

In theory, you should be able to locate your Quicken data file in a shared location (Macintosh HD > Users > Shared) and have all users able to access it there, but I I've read reports from users of problems with trying to work that way. The problem stems from he fact that the Quicken data file is not just a file; it's what's called a package file, which is a wrapper around a whole group of files and folders to allow it to look like a single file -- and the specific user permissions for all those files and folders gets messed up if the file is moved between different users of the Mac, or different Macs. That's why we say you must compress the data file, which puts an impenetrable outer wrapper on the file, as either a Finder-created compressed .zip or a Quicken-created .backup file.

But you can use that Shared user space for the compressed (.zip or .quickenbackup) file to be passed back and forth between your two users. After each user is done using Quicken, create the compressed file and move it to the Users > Shared folder. The next user to come along (whether it's the same user or the other user) starts by copying the compressed file to their own desktop, and double-clicks on it to launch and use Quicken. And the end of the session, create the .zip or .backup file and copy it back to to Users > Shared, replacing the previous version there. Moving the compressed file in his way will protect the file and folder user permissions inside your Quicken data file so it can be used by ether user.

And, of course, keep extensive backups of your file just in case anything goes wrong. (That advice applies to anyone, not just someone moving a file around like this. )