I've read reports of people who have successfully mixed up partial quantities of commercially prepared developers. I would not recommend it. There is the argument that the ingredients can settle out into layers, so that you won't necessarily get the right proportion of each ingredient in each sample. That may or may not be the case, and I have no way to prove it. It is, however, a plausible argument.
But there is an overriding reason not to do this; one that I discovered by accident with a package of Dektol. Both D-76 and Dektol are MQ type developers using the same reducing agents, metol and hydroquinone, in different proportions. Upon opening a package of Dektol, I found that the color was off. It had gone dark. When I mixed it up, the resulting solution was the color of brewed coffee. Upon examining the package, I found that there were a couple of pinholes that allowed air and moisture to react with the contents. The developer had oxidized.
When these mixtures are packaged at the factory, they are sealed in an air and moisture proof container with an inert gas. Once opened, that seal is breached allowing air and moisture to reach the contents. The bottom line is that the dry powders won't last long unless you keep them protected from the ambient atmosphere.
I know you want to do this to save money. The price per unit of measure is very attractive when buying bulk quantities of this stuff. But in the end, you'll probably wind up loosing those savings through spoilage.