A33Zab can follow up if he wishes, but I think I can answer this.
First, there are two heaters in the pitot tube, one for the pickup barrel of the tube, the other for the watertrap area of the tube. The reliability of the pitot tube heater elements is very high. The current is monitored by the control computer. You want to know if there is a heater malfunction in either heating element. When in series, if either fails open, you know about it in a timely manner with a single current sensor. Since you have redundant pitot tubes, a single failure is no big deal by itself.