It is not yet clear whether the shareholders would wait until any internal investigation is finished before taking further action.
Meanwhile, it has emerged that Magnier and McManus asked United why they are still paying Rio Ferdinand despite his eight-month ban for failing to take a drugs test.
The question was among 63 the pair demanded answers to in a series of letters.
It said: "Are we to understand the board has authorised the continuation of his full salary (which we understand to be £70,000 a week) as if he is playing while he is, in fact, suspended?"
It is believed they did not receive what they consider to be a satisfactory reply.