I do this all the time. I have an iMac (desktop) and a MacBook Pro Retina Display.
I divide projects into two categories: completed projects and active projects.
All active project are kept on 2 TB Western Digital USB 3.0 drives. Those project are managed libraries...that means the RAW images are stored in the Aperture Project Library on the external drive. When I want to work on it on my desktop, I plug it into my USB ports on my iMac and I am up and running. When I travel, I plug it into my MacBook Pro Retina Display and I am up and running. I back up this working drive daily when I am on my desktop iMac. I have found that I might have 2 or three 2 TB passport Western Digital drive that have active projects on it. Some clients simply can not decide what they want. Each of these are regularly backed up. I have used SuperDuper for this back up process, but am moving toward using Carbon Copy Cloner.
ONCE a project is completed, I move it to my desktop computer by importing the library into my yearly master library. I then relocate the master files (now they become referenced) to an external 6 TB Thunderbolt drive into a file structure I would be happy to describe if you are interested. The yearly master library resides on an internal iMac drive. For example, I have 2011, 2012 and 2013 master libraries residing on my iMac internal drive, but all the image files are stored on a 6 TB Thunderbolt drive. The external thunderbolt drive is backed up and archived. The internal drive that holds the libaries is also backed up and archived.
I don't reuse my working drives (2 TB WD Passport drives), but save them as they are filled. In the final analysis, I have 4 copies of my RAW files
1. 2 TB Passport Western Digital Drive
2. On my primary long term iMac storage drive
3. My back up of my primary iMac storage drive
4. My archive of my backup drive of my primary iMac storage drive
The drives that serve as my backup to my working passport drives eventually get recycled as I need the backup space.
The critical aspect related to your scenario is I use my Aperture Libraries on external USB 3.0 drive that I can plug into any computer I need. At this very moment, I have three such drives plugged into my iMac. Next week when I travel to my cottage, I will take these three drives with me and work seamlessly. The BIGGEST risk is I do not have a daily back up strategy or my working files when I am traveling, but I do believe there is a long term solution to this issue as well.
This is a long winded response to say I agree with Kirby and somewhat simpler than William's solution, I think.
Please ask details, but this is a very simple problem to solve.