As I am going to do cross-platform development, I can only see this is a prethinking based on my observation to other such teams.
1. You still need to work on Windows in major time if you are familiar with windows in long time. Of course, if you have been using Mac for a long time, you can choose Mac as your major work OS. Whatever, pick up your most familiar OS as your major work OS to reduce unnecessary efficiency reduction due to OS switching..
2. Keep in mind that OS specific API/Library basically should not be used, although for example, using boost library will benefit developers to write cross-platform code (or using QT to develop UI), but we should know that internally OS specific APIs will be called in the library cross-platform API calling.
3. UI now can only have one style for both windows and Mac, while of course the look of the app window or text box should be different in style natively for specific OS.