Apparently in 2009 Obama visited Moscow and managed to meet both president Putin and president Medvedev. Granted, it is sometimes hard to remember who of these two is a president at any given moment, but a two-president state is a bit pushing it.
http://emilypost.com/advice/addressing-a-former-president-of...
https://www.whitehouse.gov/1600/presidents/williamjclinton
Where I live, Finland, a "President" is the official title for former presidents (there are now three alive) so I should actually address them as such. The current one is formally referred to as "President of the Republic". I can't really check how this is with Russia.
Neither of them new protocol for the ex-presidency thing and after President left office, upon mutual agreement neighbour now calls him "Uncle".
Met with then-President-now-Prime-Minister Medvedev and former-President-then-Prime-Minister-now-President-again Putin?