The reason why a man can suffer an electric shock in any situation is because a small current passes through his heart and body. The current requires a complete path to continuously flow, and in order for the electrons to flow through a conductor, there must be a voltage present to motivate them. It is an amount as small as 100 milliampers or less that can kill any person.

Considering the ungrounded situation that you describe for the man, here you have the answer for each case:

Part a: As the man is simultaneously touching two cables with the same or identical voltage and polarity, there is no path for the current to flow. You have the same voltage in both cables, there is no ground, and then there is no way for the electricity to circulate through the mans body. I make it clear that the voltage in both cables has to be exactly the same; otherwise a current will flow through the man and will harm him.

Part b: We assume again that the man is not touching ground. But the cables somewhere are grounded. If the man touches at the same time two cables with different voltage each, there will be a resulting different voltage, so a current will flow, and it will find a path through the man- to the ground. In other words, the man will close the circuit and he will be seriously harmed.