No, not yet. I've been getting home from work too late and he's been closed when I get there. I'm hoping to stop by tomorrow (sat).
In speaking to someone else about this, he said that he is 99% sure that you can change from singles to duals, I asked him to back it up and we did a few minutes of internet searching and couldn't find anything conclusive. A number of forums said that when they went to a shop, the shop said no (I had the same experience at a Meineke). But no one came up with anything offical (epa, gov, etc.) to back it up either way.
My searching came up with this so far;
http://www.epa.gov/otaq/cert/factshts/catcvrts.pdf
In it the only thing I found in it that comes close to to an answer is this on page 5;
"Install the converter in the same location as the original."
So that could be subject to interpertation, it doesn't say I can't add a second as long as one of them is in the same place as the original.
"If you ain't cheatin', you ain't tryin'", Junior Johnson