Dual radio (same band) AP's are quite uncommon. I actually use some in my WISP and haven't noticed a throughput problem, but we don't really overtax the channel so I may not have noticed it.
If you have one of these dual band AP's you could test it. It would be great to see your results.
With our dual radio same band AP's we deploy these on a sector array so the antennas have separation. My question is, would the fact that both radios are in the same box just inches from each other cause problems like is being described in the paper that the OP referred to?
In my humble opinion, dual radio 2.4 band AP's are practically useless. It is possible to see an advantage, but in most environments we want AP separation. Now, I do believe that there could be merit to a dual radio 5 GHz band AP. With so many channels to choose from you could easily design it to have no channel overlap and have to run less wires (big cost of installation).