Being able to use more than one element at one time doesn't give you the ability to wield a KKG or KKT. Using a KKG and KKT involves fusing elements at the level of their chakras, not their physical forms (eg: Wind and water creates an Ice KKG, but when a wind and water jutsu is combined, it doesn't mean an Ice jutsu results!/ eg: They physical forms of the water and earth elements creates a sort of muddy water jutsu, but when combined at the chakra level, they create Mokuton!). Non-KKG wielders do not possess the ability to combine nature affinities at the chakra level.
Simply put, to make cake, you can't just add flour and eggs to water; you need the right temperature, moisture, etc... the KKG/KKT provides the genetic ability to provide these auxiliary conditions needed to combine elements to form advanced elemental techniques, such as ice and lava.
Going by your logic, most characters in the show would be able to use KKGs, BUT they can't, because having multiple nature affinities DOESN'T grant you the use of a KKG; conversely having a KKG grants you the use of multiple nature affinities...
I hope that wasn't too confusing...