A researcher cannot participate in two MSCA Postdoctoral Fellowship grants at the same time.
On p. 22 of the Model Grant Agreement it is specified that the recruited researcher must work exclusively on the action unless part-time for professional reasons has been approved.
On p. 12 of the MSCA Financial Guide it says that part-time work may not be requested to participate in another MSCA grant at the same time.
Moreover, on p. 15 of the MSCA Financial Guide it is stipulated that a researcher cannot benefit from two Marie Skłodowska-Curie actions at the same time.