Can I Split an Automated Backup Across Two Portable Drives?

This is not possible as far as I'm aware.  I believe this is a limitation of most home backup products as it only allows you select one source and will always stry to use that space.  

You could technically move the incremental files to another drive (manually)  and then "validate" the backup.  When prompted that files are missing, you could then tell it where those files were moved to (you have to do this for each moved file) when it says they're missing and asks if you want to locate or delete them.  However, I think you'd have to continue to manually do that on your own as new backups would still be written to the original drive again.  I'm also not sure how reliable the backup would end up being going this route, but I don't see why it wouldn't work if you're were willing to do this manually.  

Typically, you want your backup drive to be at least 3-4 times larger than the orginal to give you the ability to have multiple versions of backups or increase the retention of backup chains.  

Dale,

I agree with Bobbo's analysis.  I also believe that moving files around would be risky, especially if you need to perform a restore operation using rescue media.  Also, you will have to delete your only backup prior to starting a new backup chain.

If you are able to buy a bigger hard drive, then you could still use your existing external drive for periodic backups...then disconnect and store in a safe or off site.