I am in the business, and do sell to the public. I can give you a hook up. -but- that is not the reason for the post. The filters are one way, there is no fear of them back flowing into the filters and causing drama. You are correct however that when you want to paint, close the one ball valve and double filter the air. If you want to sand blast and run tools go thru shut the other valve.
IMHO, the double filter is over kill. If you choose the right filter, your flow will not suffer and you will have clean dry air to paint or run tools. It will save you money as well since you do not have to by 2 units.
check out this link:
http://wilkersoncorp.com/
Darren
Let me know if you need more help.