Multiple boards blinking at same time
Have both sync to a master clock using GSM shields. These shields are getting very cheap (as low as $14 on ebay).

The shields receive the time wirelessly from the cellular network and this time base is extremely accurate. Here is some example code to read the current time from the shield...

http://forum.arduino.cc/index.php?topic=163155.msg2102141#msg2102141

Note that you will need a SIM card to get the time, but you should not need an active cellular account.