I meant multiples of the same, but it applies to either. I know many people use a single stat attached to several heat sources (usually mats in racks). In the majority of cases, this works fine, but the thermostat is only really controlling a single mat. If one of the mats that doesn't have the probe nearby overheats, then the thermostat will not switch off the power to that mat. It could even catch fire and the thermostat would be unaware of this.
What usually happens in practice is that people use several mats which are similar enough that they all heat to the same level as the controlled mat. If an object (such as a snake) is placed above one of the uncontrolled mats, though, the heat doesn't dissipate as quickly as the controlled one and it can overheat.
As I said - most people don't have a problem with this, but I don't like to see it recommended as good practice, that's all.