P0299 is typically a broken wastegate actuator (2.0) or blown seal (3.5), meaning new turbo time. IDK if only 1 has to be broken to cause the underboost or both. I don't know enough about the balancing act here to speculate ... There isn't any individual control over the turbos, just a control over the overall system, that assumes the turbos are working the same and responding the same. Probably Ford thinks it is enough to point to the probable rather than the exact culprit with the code.