Would you say that you have to use two different kinds of “bacon” (pancetta/guanciale) when you make carbonara/amatriciana?
No, normally you'd use one or the other.
Both preparations are from the Lazio region of Italy where guanciale is a regional speciality. Thus, guanciale would be the preferred cured meat.
Pancetta is a more widely produced and non-regional specific product and would be the substitute if guanciale is not locally available or cost considerations make guanciale prohibitive.
If a given recipe lists both meats, it is likely that the author is trying to reduce costs. In my opinion, there is no qualitative benefit in combining the two meats in these sauce preparations.
Not at all. For years, there was only pancetta (I'm in the US). So that's what I used and it didn't affect the carbonara or amtriciana in the least. It's still difficult to get guanciale. Use whichever is available in your area.