Systems and methods for transeptal cardiac procedures, including adjustable, separable guidewires

A method for treating a patient, comprising:

• deploying an intravascular guidewire between two layers of tissue, the guidewire having a first branch and a second branch fixedly secured relative to the first branch at a first location and releasably secured relative to the first branch at a second location spaced apart from the first location;
• controlling a distance between a portion of the first branch and a portion of the second branch with a controller positioned outside the patient, while the branches are positioned within the patient with the second branch releasably secured to the first branch at the second location;
• performing a procedure on at least one of the two layers of tissue in a region between by the first and second branches, including at least partially fusing the two layers of tissue to each other by applying energy to the two layers with an energy delivery device;
• separating the first and second branches from each other at the second location;
• moving the first and second branches relative to the layers of tissue while the first and second branches are separated from each other at the second location;
• and
• removing the energy delivery device from the patient's body.