When he takes over the intelligence service ISI, Lt Gen Rizwan Akhtar will become one of the two most powerful men in Pakistan, answerable only to the army chief.
I don't think that this the case. Both Army chief and ISI director are independently powerful and neither report to each other. Case and point, Kargil fiasco and the 1999 coup.

Ziauddin has served as Director-General of Pakistan's premier intelligence agency ISI. He was nominated for the post of Pakistan Army chief on 12 Oct 1999 by then-Prime minister Nawaz Sharif after the dismissal of General Pervez Musharraf, who had begun a coup against the government.
http://en.wikipedia.org/wiki/Ziauddin_Butt



Also, here's a discussion between Hamid Mir and Indian journalists. Couldn't find the relevant section so I am posting it here.

http://www.youtube.com/watch?v=tMn6otJ0VP4