Hi dsttexas.
You could plug them in as you want to use them. Then open N360: Backup: Manage Backup Files -
Then look at what it says about your standard backup set.
Does it show that the location it expects to back-up to is the disk you want? If everything looks fine, then you could try it and see. If it is showing anything odd then it would probably do a new full back-up.
If N360 thinks there has been a major change it may do a full backup but I know of no way to be sure before it does it. If anyone else does - please say so. The two worst aspects of that would be the time it would take and possibly the space it takes. The space is a "possible" as sometimes I have seen these extra full back-ups being added to the size of the original file. It might just replace the old backup.
You could also try a test when you have your two machines together. Set them up as you normally have them and create a new test back-up set. Put a limited number of files into it to keep the processing time down. After it has run successfully once, change a couple of the files and run it again. This should produce a very quick incremental backup. Now separate your two machines and set the disks up as you would when traveling. Again change one or two of the test files. Now run the backup again and see if it runs an incremental backup (as you wish) just processing the changed files or, if it runs a completely new backup.
Once you have run the test, you will know the answer to your question and you may delete the test back-up set.
Does that help?