Hello my name is XXXXX XXXXX I will be working with you today. I believe what you are referring to is the first base running lane, which is the only double baseline.
Rule of First Base Running Lane
1- The batter-runner is required to run in this three foot zone, the last half of the distance to first base.
2- Both feet must be in or over the 3' zone, one foot in and one foot out is not good enough.
3- If both feet are in the lane; but the throw hits the runners' arm, which is outside the lane, there is no violation.
4- If the runner is hit by the throw or a collision occurs on his last step before touching the base, generally interference is not called. The runner has to step into fair territory to touch the base, which is in fair territory.
5- The batter-runner is out if, in running to first base, he is hit with a throw while he is running outside the 3' running lane, or interferes with the fielder taking the throw at first base. He could be called out even if he is not hit with the throw, if the umpire judges that by being outside the lane he interfered with the fielders attempt to field the throw.
6- There must be a throw before interference is called and the throw must be a quality throw.
7- The baseball rule states that he is out when out of the lane and causes interference with the fielder taking the throw.
The Parallel First Base Line Exists:
to provide the umpires with a visual guide with which to make a decision.
It is also there as a guide for the batter-runner to know at what point he
must be in that running lane, so as not to be called for interference. The
line begins half way to first base, distance dependent upon the
dimensions of the field.
Please let me know if you have additional questions.