Windows vs Mac software options - allocating splits

First, understand that Quicken Mac and Quicken Windows are cousins, not identical twins. The software is different, the data files are different, and they aren't interchangeable. There is a one-way conversion process from the Window to Mac version. 

Second, nothing has been "dropped" or "lost" from Quicken Mac, but there are features in Quicken Windows which haven't been built (yet) for Quicken Mac. An earlier generation of Quicken Mac software existed up through 2007, and then the developers started over because the technology it was built on could not be updated to run on the evolving modern Mac operating system. So they started over, and came out with a new Quicken Mac in 2015; it was pretty bare bones, and they've been working on adding functionality to it ever since. There are more than 600 user requests on this site in the Product Ideas - Quicken Mac section for new or enhanced features, so the developers have to pick and choose which ones to work on to have the greatest impact on the largest number of users. 

In fact, there is an Idea post for this feature from Quicken Windows which you can view here. You should click on that link and add your vote for this feature. (I'd note that that particular idea has garnered less than a handful of votes so far, which helps explain why it hasn't made it onto the developers' action list yet.) Additionally, you may want to add your vote to a slightly different but similar feature from the legacy Quicken Mac product for setting up sales tax amounts which could be applied with a keystroke.

I'm not sure I understand your last question: "If I get the most recent Windows Quicken version and run it in parallel on my mac, will I have this automatic split functionality?" Are you referring to Parallels, the virtual machine software which lets you run Windows and Windows programs on your Mac? If so, then you're simply running Quicken Windows, with whatever features there are in that program.