Welp... with some extra searching I just found the answer to the registration deadline question over on CNET...
http://www.cnet.com/how-to/how-to-upgrade-to-windows-10-for-free-after-the-july-29-deadline/ Essentially I'd have to download, create the ISO image(s), install and register Win10 on both PC's. So effectively the upgrade and registration MUST happen now. Then I'd have to use the "Rollback" function to revert back to the previous Windows version.
At that point I could use my current version of Windows 7 for as long as I want then install Win10 whenever I feel like it.
Problems: This means actually pushing through the upgrade and rolling back which supposedly can cause problems with drivers and programs (not supported by Win10) so those would need to be fixed after the rollback (if any arise). The article states this seemed to mostly be an issue with Win7 installations.
A workaround could be to install on a partition to register the Win10 install instead of borking out the Win7 installations. However, due to the deadline and my technical ineptitude (and having other things on the go) I don't think I'll be able to muster the brute force to create the partitions, do the downloads, create the ISO's, register the products, etc on both systems before time runs out (already kind of burnt from meatworld mayhem today and the MS servers are likely going to be more than useless due to the increased traffic).
Oh well. No bigs. I like Win7 (and am skeptical about Win10 which is why I wanted to test it) but maybe this info will help others freaking out.
Cheers, all. Happy nerding.
;-)