BinghamtonNY / Ryan
Yes there are two 8 inch ducts going into the plenum. On my post I link to, I have pictures of the finished project.
I answered my own quesion last night, and found what the pros do for installing hot air side by side. Look at this link, http://www.harmanstoves.com/doc/pf100m.pdf
This link is broken, either the page no longer exists or there is some other issue like a typo.
. See figure 33 on page 17. They run them hot side into hot side, cold side into cold side. They speak of an isolation damper. I tried to find one, but didn’t find one a quick search.
You might be fine with your oil burner running into the cold return, I've heard of people doing that. My gas furnace is a 92 efficient model with lots of fine circuitry. The manual says to use wire rated at 110 degrees if I need to replace anything inside, and that worries me blowing 130+ degree air though my unit. The PDF link says you can damage your furnace if you connect it to the cold air inlet. If your furnace just has old relays and valves, you can probably do it. Just don't burn up your furnace trying to save a few bucks.
If I'm burning coal, I'm trying my hardest not to let that furnace kick on. If it does kick on, the air is slowed down quite a bit by the fans... and what does get by just warms up my basement a little. The house still heats up pretty fast still. In the spring I'll put a manual damper in when the fires out so there is no back flow. This is all new to me, so I'm learning too.