Help me figure out an Aperture Library Solution

I have a Macbook Pro and an iMac with Aperture on both. I want to have one library where any changes that I make to the photos or library on the iMac are also made to the photos or library on the Macbook and vice versa.

I'd prefer not to have to carry around an external drive with the Macbook.

If referenced files on an external drive is the only solution, how do I continually back up the masters on the external drive?

I've been sifting through hundreds of posts on this subject but can't find any that are specific to my situation.

Thanks in advance.