Yes, you can make changes to the catalog on your portable drive while travelling and later on merge that catalog with the main primary catalog on your external drive.
Chetna has you pointed in the right direction for working with a master and "travel" catalogs.
Alternatively, you could put your images and catalog on the external drive and just move the drive from computer to computer as needed.
I have a similar issue as Jim, I have a backup of a months workload and within that backup 2 identical cats...however sometimes I opened one or the other cat to work on photos...so then my guess is they are no longer identical and my fear is if I merge the "identical" cats I may lose some of my work flow for some folders. When merging cats that are "fairly similar" will it preserve all workflow for both upon merging?
Thanks Chetna, hopefully my terrible complex explanation made sense haha