+1^
Safe yes.
But I think not a great idea. It is harder to manage two pools of space.
If one has fewer nand blocks underlying, it will get more than it's fair share of updates, leading to premature wear. But.... wear on a ssd is really not an issue.
And... to what end?
Reinstalling windows in a partition sounds fine, but you will need to reinstall apps that go onto the registry also.
FWIW, my method for backup is to occasionally clone my ssd to another.