Sometimes a sensor can malfunction, or the area surrounding it can be infected, leading to wrong glucose measurements. Even just the fact to rely on a single sensor seems to psychologically limit the adoption of such devices, both by physician and patients.
Naively, using multiple sensors located at different places looks like a reasonable solution.
Why do most (all ?) insulin pumps use a single glucose sensor ? What are the reasons not to use multiple sensors ? Anticipating on the "sensors are expensive" answer -> They are mostly because of R&D costs. So once developed, using several ones should not be a lot more expensive than using one.