I think it would be preferable to use a tool that has the expander rigidly located as the first step, then run it through the .284 die to tidy up a tad.
I'm running a 30/284 & I expand up either with a K & M 6mm to .30 expander (the one made for the 30 BR) or in in two steps, first taking it up from 6.5 to 7 mm with a RCBS expander die that I happen to have lying about.