Can a creature (or a character) have both of these templates?
Fiendish seems to apply to "normal"infernal denizens. Like a fiendish...cow for example.
while half-fiend seems to be a more drastic template but only the fiendish one gets dr/good
If you're the GM, you can do anything. You can make a Lich Vampire, or a sorcerous toad, or an advanced baby half-dragon monk, even if it's not legal. So the answer is no, but yes.