The Runtime engine is platform specific. You would need to create the runtime on a Mac for your Mac users and on Windows for your Windows users. You can user the same Filemaker file to create both runtimes as long as there are no platform specific scripts steps. If there are you'll need some conditional scripts to deal with it. Also be aware that fonts render differently on each platform so you'll need to do some testing.
I have a MacBook Pro with Parallels so I can also run windows. I have Yosemite, Windows 7 and Windows 8 all on the same machine. Filemaker comes with installer for both MAC and PC, so then you can make both runtimes. You need to test your solution on both platforms because even though FM runs on both there are some difference. Fonts display different on both platforms, items that use the web browser such FMURL and Webviewer. PC uses IE and Mac uses Safari.
My solution is already tested on a Mac computer (but with a Filemaker Pro installed on this user's computer). There was no problem at all with fonts and scripts.
So it is mandatory to use a Mac computer to be able to do the Mac runtime.
Thank you for your help.
in addition to schamblee's scenario which is same as I deal with:
I do programming on the Mac OS. Since Parallels can reach Mac resources from any Windows VM, you can do Windows Runtime-Binding with the Mac resources.
Make sure you set same Binding-Key for both Win & Mac Runtime!!!
(your customer may want to switch from PC to Mac or otherwise later)
I recommend to have one folder for the RT-Binding and another folder with the final resources. When doing updates, you copy only the edited files.