I know an individual Metro ticket is valid for 2 hours of travel/transfers.
Does that mean if I take a quick trip, leave the station at my destination and then return to the same station for the return trip all within the 2 hour period, I can use the same ticket (1 ticket for the entire trip?)
1 answer
No, transfers can't involve exiting the Metro/RER station fare-paid area and later returning to the station to continue or backtrack the journey. The Paris Metro (ticket t+) / RER ticket will not work to open the turnstile/gate to get back into the station later in its 2-hour validity period. (Transfers aren't even possible/allowed after 90 minutes of the 120 minute validity period have passed.)
There are handful of transfers within central Paris that do allow exiting a fare-paid area and entering into a different station that requires street-level access, but these are specifically programmed, popular transfers from a certain RER station to a certain Metro station for which a connected, underground tunnel doesn't exist. But these are rare.