As far as i have understood, Tree has a direction from top to bottom, and if T2 wants to lock both G and H then it has to lock the common parent of both first, Here T2 is not allowed to request a lock on G, because it can't climb upwards. Now T2 can request lock on J only and not on any other nodes.
"The first lock by Ti can be on any data item", that time itself T2 should have taken care before it locked H if it needed G and H both.