OK So, I found another post with a user having WoL issues. I'm pleased to say that I learned the Virtualbox network adapter(s) interfere with WoL (with the exception of Magic Packets).
After uninstalling Virtualbox entirely, I was able to WoL using Multiplicity. Unfortunately now I have to figure out how to make Virtualbox work with Multiplicity as well. If anyone has the how-to, please let me know.
I know the whole spiel "take it up with Virtualbox support" comes to mind, but I'd appreciate this being worked from both ends if possible.
I do actually use Virtualbox for some things, including testing Unity games/apps on multiple platforms. I'm not making any AAA titles but I am aspiring to be productive with these tools.
Any assistance would be appreciated.
Additionally, when I make the secondary machine sleep using the primary's mouse/keyboard, the mouse becomes stuck and the cursor just dies. Until I reboot the secondary machine entirely, the mouse and keyboard do not function properly and the mouse stops responding entirely. So, that's a separate issue to address.
PSA I did install the beta on both machines, which is what brought my attention to the WoL/Virtualbox issue.
Thanks @basj