"The only stupid question is the one that isn't asked"
Flatbeds on single rear wheel trucks are no problem - 2 of the 3 welding trucks at my work run singles.
As sts points out, the duallies are mainly there to increase cargo capacity. Depending on how wide the flatbed is, it might LOOK a little odd with single rear wheels but as for installation/general use - no problem. Just keep in mind that your GVW (Gross Vehicle Weight) stays the same regardless of what style box/deck/body you go with.