I’m 90% (I leave that 10% just in case) ;), that the Wi-Fi card that ships with a Mac Pro’s is not capable of sharing the ethernet connection (that connects to Internet) over the Wi-Fi in dual band mode. The reason for this is that the very same Airport Express is not able to do that (and it’s a dedicated piece of hardware). You can infer that from two things:
It’s not mentioned in Apple’s website that the Airport Express (or Mac Pro) have simultaneous dual band, which would be required for what you want.
It is mentioned that (and I quote): “Speed and range will be less if an 802.11a/b/g product joins the network”. That means, that although they can use either 2.4Ghz or 5.0Ghz, they cannot use both at the same time, and when a slower 2.4 device (using a/b/g) joins, the antenna must switch to that speed, because they only have one antenna.
The Airport Base Station (Airport Extreme), on the other hand, mentions that in the very front page: “AirPort Extreme operates in the 2.4GHz and 5GHz bands simultaneously and offers even better performance and range.”. Seeing is believing ;)
So the answer to your question is: No, the Airport Extreme card on a Mac Pro (or any other Wi-Fi Mac Computer for the matter) as of November 2010, is not capable of using a Wi-Fi connection in simultaneous dual-band; not for sharing, not as a client either.