Thanks for the question. I've got a combined 8.8 gig available (Strictly Business + 2 Code Monster) but was getting charged for being overquota for being wildly overquota on one plan.
I've always had "one" bill from Dreamhost, but my services have not been aggregated like I think you're asking.
In the meantime, I did what support said and "moved" it over to another domain on the other "plan"; fairly painless on the move. But now I have to repoint static html to the "new" location for it to be visible since that was before I got into PHP/MySQL. Woulda been much easier to make an accounting change than for me to do what I did. But, now I got my answer that it is possible.