There is no need to mix the malts if they are going to a small silo/grist case. The nature of the silo plus the mixing in the mash will create a pretty good blend. I have observed that in dark beers that no matter when I mill the dark grains they always seem to come into the mash throughout the mashing process. For simplicity's sake mill your base malt first and then adjust your mill and do specialty's last I would chase the last bit of specialty malt with enough base malt to ensure the auger doesn't hold any specialty malt that might affect your next brew. The nature of the silo will pull the specialty malt down the center as your mashing giving you a good mix. Check your grain after grain out on a dark beer and you will readily see how ell the dark grains have mixed in.
"You are what you is." FZ