21.—Where any one or more of several persons entitled to any land as F14[…], joint tenants or tenants in common have been in possession of the entirety or more than his or their undivided share or shares of such land for his or their own benefit or for the benefit of any person or persons other than the person or persons entitled to the other share or shares of such land, then, for the purposes of this Act, such possession shall not be deemed to have been the possession of such last-mentioned person or persons or any of them.