Can two Macs be kept in sync using a single external hard drive?

To keep all settings and files of two Mac's in sync through an off-line method you could place your Home folder on an external HDD. There are plenty of tutorials on-line on how to do this, I will use this one to explain the process.

I assume you have a user account Allan on the current MacBook Pro (MBP-1) which you would like to share with the new laptop (MBP-2) through an external HDD named ExtDisk. Keep in mind that you should substitute the actual hard disk and user account names.

1. First create a back-up of the user account in case something goes wrong. Time Machine is an option for that, but you can also clone the drive with for example Carbon Copy Cloner.

2. Create a new administrator account on each MacBook Pro. This is for the case that you lose the external HDD for some reason or just don't have it with you. This way you'll still have an account from which you can use the laptop.

3. Migrate the Allan user account from MBP-1 to the new MBP-2. Here's an Apple Knowledge Base article on how to do that.

4. On MBP-1 login to the newly created admin account. Copy the Allan home folder of MBP-1 to ExtDisk. You can do this by using the following Terminal command (the Terminal application is found in Applications -> Utilities -> Terminal.app):
   sudo ditto -v /Users/Allan/ /Volumes/ExtDisk/Users/Allan
   This command will ask your admin account password to get permission to copy all your files to the external disk ExtDisk.

5. When you're done copying, open ExtDisk in the Finder and check if the user folder is there and has (roughly) the same size as the Allan user folder on MBP-1.

6. Go to System Preferences -> Users & Groups. Right click on the Allan account and click Advanced Options....

7. The Home directory line says now /Users/Allan. Click Choose... and navigate to the home folder on ExtDisk.

8. Connect the external HDD to MBP-2 and repeat steps 6 and 7 on MBP-2.

9. Do the following on each Mac with the external HDD connected: login on the Allan account and check if everything went right. To verify that it actually uses the home folder on the external HDD, select a file on the Desktop, hit cmd+I to show the Get Info window and check if the Where field says /Volumes/ExtDisk/Users/Allan/Desktop.

10. If all went well you can delete the /Users/Allan folder on the internal hard drives of MBP-1 and MBP-2.

11. Now you can use the setup as follows:

    • plugin the external HDD and turn on the laptop
    • login on the Allan account at home
    • shutdown the laptop at home (maybe a logout is sufficient).
    • disconnect the external HDD. Go to work.
    • plugin the external HDD in at work and turn on the laptop.
    • use the Allan account at work.

Pro-tip: as you will be carrying around your HDD with you the chance that the drive has an unpleasant physical encounter is somewhat higher. Make sure to have a daily backup running, either at home or at work, or if you really want to be sure at both places.

Disclaimer: I have not carried out this process myself. I don't accept any responsibility in the case you lose files.