since i have not yet built the system (havent even bought parts aside from case and PS), but will buy the drives first, can I use them for backing up data BEFORE building the desktop and setting them up for RAID1?
in other words: do I need to format both drives when setting up RAID 1?
or can I keep data on one and have the data copied onto the other drive in the RAID1 setup process?
This depends on the specific RAID implementation. In many cases you can go from a single drive to a RAID1 and keep the data, but it is not a sure thing. I would not count on this working, even if your RAID of choice supports it.
my question is: will both operating systems be able to access and write to the RAID1 array? or do data drives need to be formatted differently for each operating system?
It would be quite difficult, but not impossible to share a RAID between Windows and OSX. Software RAID is probably out as are likely most "fake RAID" controllers. You would probably need a real hardware RAID card that is compatible with both Windows and OSX. This would add a fair bit to your build cost and otherwise be a waste being used for just RAID1.
After the RAID hurdle you would need to use a compatible filesystem. For modern OSes the best choice is probably exFAT. NTFS would probably also work. FAT32 is the most compatible, but it would be unwieldy on any decent sized disk.
if above is not possible I may skip on the idea of going dual boot/hackintosh but I thought it might be worthwhile to try since ive never had a mac and primary reason for this build will be a DAW and video editing system.
If you are going to do any serious work then I would skip the hackintosh or just have it around for fun. There are lots of good DAW and video editing programs for Windows.
If you really want to go Windows/hackintosh dual boot then I would highly suggest you skip the RAID. Use one single drive internally formatted as exFAT or NTFS and keep the other drive as an external backup.