light up thread 1 , when thread 1 is half done light up thread 2 , and make sure you mark the half of thread 1 , now when the other half of thread 1 is half done you have reached 45 minutes point countdown, now countdown your 45 minutes on thread 2( the remaining of thread 2 will last for 45 minutes.) or you have just reached 45 minutes from the time you lit up thread1.