I do not believe you can use Fat32 formatted volumes to make a software raid mirror in any version of Windows, ever. If I'm wrong, tell me, but I have been working with the 'Disk Management' in windows for 15 years and have never been able to do this. Not that I would.
'For portability' is not a good reason to use a file system that was left behind by Microsoft years ago. You can mount an NTFS partition in several O/S's these days effortlessly. I agree with every one else here. Build hardware RAID 0 into your system, use all the space on those disks that you paid for, get some performance out of them. And most of all, PRACTICE RECOVERING YOUR DATA. Do mock-crashes, do some research, READ what other are doing about their problems, make some notes, get acquainted with your hardware, and your backup software, and whatever tools you end up using for data recovery. This is essential for real data loss prevention.
Am I preaching? Sorry!