I am not an L&L employee, but my understanding is that the only times there have been a discount for buying licenses for two platforms is when they have offered a bundle. So buying the direct-from-L&L PC + Mac (current versions) is a bundle that is a little bit cheaper than the separate cost of the two licenses.
However, I do not believe they allow you to do a cross-platform “upgrade” like this. You simply buy the new license for the new platform. And right now, if you buy the current Windows license for Windows 1.9.x (which is file format compatible with Scrivener 2.x for Mac) you will get the upgrade to Scrivener 3.x for Windows for free once it is released. And since the Scrivener “full” cost is still extremely reasonable, I hope that meets your needs.
HOWEVER, act fast. Like today. We are in the process of changing licensing vendors, and will not be able to offer the cross-grade discount in the new store. More information on the switchover here: literatureandlatte.com/blog … ith-paddle