All at the same time, light both end of string#1 and light 1 end of string #2
String #1 will flame out at 30 seconds, somewhere along its length. At that instant, lite the remaining end of String #2, which has 30 seconds gone, and 30 seconds left to go.
String #2 will flame out 15 seconds after you light its second end.
I'm here all week. Remember to tip your waitress.