The easy way to keep both phones the same is to get a program such as nvbackup (just run a search there are a few quality freeware options out there) Which will backup your entire phone settings and programs onto your sd card. Install the program to your primary phone as per instructions, and perform a backup using nvbackup (or whatever program you decided to use) to your sd card.
Then swap the sd card to your reserve phone and perform a restore from the backup you just made.
Keep in mind that your 2 phones will now be identical to use. any previous differences on the reserve phone will no longer be there (so if you like you can do a backup on that phone first just in case, labelling it so you can tell the difference).
There you go, easier, less steps, and faster than backing up to your pc

Doing this isn't too complicated, though it can be a bit frustrating to keep them straight.
The safest thing to do is backup your primary phone during a regular sync.
1. Once you have performed and completed this, then open your Palm Desktop.
2. Look to the upper left hand tab marked "Hotsync" and click it.
3. On the drop down menu, click "Custom."
4. Here's where it turns into a bit of a process...you must now set each of the sections in this list to "Desktop overrides Handheld." To do this highlight each one then click "Change" once you do this you should have the option to override your handheld. If it doesn't appear then you simply do not have this option available. When you've completed all overrides, click "Done."
5. Click "Sync your back-up Treo as you normally would. This should make your backup identical in information to your primary.
--------------------------------- 6. Once you are ready to go back to your primary, You must go back your Palm Desktop and enter into the "Hotsync" tab and then "Custom" again. This time, however, make sure you select "Handheld overrides Desktop."
7. Once this is performed, you may hotsync your backup to your desktop.
8. After the hotsync, go back to the desktop software a final time. Go to the "Hotsync" tab and then "Custom" a final time. In each selection, choose "Desktop overrides Handheld." Click "done" when finished.
9. Finally, perform a final hotsync with your Primary phone. Once completed, it should have all your original information PLUS any new information you placed on the back up phone.
Again, I know it isn't an easy process...it isn't as hard as it sounds...it just takes some getting use to! You can do it!
I hope this helps!