I got 2016 for Mac through Microsoft HUP this year.
I really wish one key worked for both, but then again, for this generation, one key only works for one install. I remember getting Office 2013 and it worked for two devices, though of course still one platform (as 2013 was only available on Windows). Now that there is kind of a parity between the two versions, it sure would be nice, but obviously I don't expect it as hardly anyone who offers multi-platform software (and allows multi-device licensing) allows you to use it on both Mac and Windows.
I am very happy that You Need A Budget actually is truly multi-platform and, for all intents and purposes, unlimited device licensing.
I think HUP is only a one-time purchase per Office version. I'll have to try at the turn of the year, because I wanted Office that was up to date on my Macbook, but I also really really want 2016 on my Windows desktop. Thankfully 2013 is still very nice.