I might not be 100% right on this. But I think a raid setup needs to use the same type of HD's or it will defalt to the slowest one.
I would do a little more research before trying this kind of setup.
By all rights's, if you are going to use RAID as a backup system, you might be just as well off with a good backup program.
If you are trying to write to both drives at the same time. I don't think you will see that much improvement, being that todays drives are running fairly fast.
RAID was a big help several years ago.
Just some thoughts I had in the back of my head when I researched it years ago.
RAID is still being used in big networks to guarantee a backup copy. But this is the kind of situation where they would be using 10 to 100+ drives.