Very good idea. I didn't know salts can be mixed like that... I've heard people switching salts that destroyed coral.You can also mix different salts. @nly04 says microbelift has a low dkH. If you mix half of that with half of your RSCSP, then you'd have new water that's closer to your tank params.
I just came across many threads (including @brleong) about rolling the bucket around to get it mixed well. I'll try this out on my next 20% PWC and see how the params are...