The difference between ISO and DIN is simple, I will generalize so that others may benifit:
*First off, you may know this already, but DIN is just a general term for standard measurement for headunits.
*ISO uses the supplied screw holes on the sides of the unit to mount it into the spefied location.
*Those units that are not ISO utilizes the cage provided to mount it to the specified location.
In general applications for a single DIN units, they are dependant on how the factory unit was mounted, or on how the kit being used determines weather it is ISO or cage mounted. Some kits allow you to have the choice between the two. Also depending on the how the Headunit is removed from the cage, ie keys, may have a factor on security, whereas in ISO applications it is not as convienant to remove the headunit with keys and may require mild to heavy disassembly of your dash to remove, adding a factor to security.
In your application, without requiring any additional or intensive modification, you are attempting to integrate 2 seprate single din units into a double din space. *The 1 double din non-ISO kit would not be compatible for your application because you propbably do not have a single cage of a dblDIN size that would fit and secure both units. I've seen it attempted this way but the results were not appealing. *Therefore you would require the 2x single din Iso kit so that you may secure each single DIN device onto the kit individually.