Actually the Executive branch of government, of which the DHS and DoD are in, has a unified classification program.
I concede that there may be some customizations per department. My access to DoD classified SCI material was suspended years ago when I left that job function. I do have some limited access to DHS classified materials. Although rare and mostly LE related, the systems appears the same to me.
Dated Dec 2009 - that's after my access ended. Obviously there is some level of common sense and logic somewhere to realize the multiple schemes were silly.