You don't usually have a sign up saying that you have an employee making sure that the other one does his job. Having a yotzei vinichnas when you have a mashgiach tmidi basically says we don't trust the mashgiach. With that being said ,its just worded poorly. They probably mean to say that there is a mashgiach tmidi along with extra oversight and inspection by one of the certifying rabbis.
Lol, every single business or store has that whether there is a sign or not. Every person on this forum understood exactly what the sign meant.
Do you run a business? Do you just hire people and hope they do their jobs without any oversight whatsoever? Oversight doesn't = a lack of trust.
Besides, the yotze v'nichnas is not just overseeing the mashgiach, they serve an extra supervisory role that is different from the temidi.
This is standard practice not just in every hashgacha (or at least any that is worth its salt).