I would never mix powders. That can be a one time only mistake. However, your statment is correct. The powder formula is the same. The difference is that the "sc" stands for short cut. The shorter cut meters much better than the standard cut. I use the 7828ssc in a couple of my ultra mags. You need that efficient metering when your dumping 90+ grains.
If you did dump them together, you would be losing the benefits of the shorter, more consistant charges.