AA vs AA

[Angel]

This is my first post and if I've chosen the incorrect forum within this site, I apologize in advance.

Twice recently, in as many days, I found myself on the river splitting the pot with Aces against an opponent who also held Aces. While it's not something that comes up often, twice in two days piqued my curiousity and for my own edification I decided to calculate the odds of this event happening once. Prior to actually picking up pen and paper I happened upon an old article by Brian Alspach (Poker Digest Vol. 5 No. 2) which stated that the probability is 1/136. He said that the math for the above calculation was in his previous article. I've got two problems. First, I don't have that issue (presumably Vol. 5 No. 1) and secondly, my attempts to arrive at the 1/136 number through my own efforts has failed. Would anyone care to assist me in this?

Thanks,
Angel