A two-way fastener includes a first member threaded for being screwed into a first workpiece; a polygonal wall for receiving a tool to screw the first member into the first workpiece, the polygonal wall located at a base end of the first member; and a second member extending in an opposite fastening direction from the first member, the second member configured to be fastened to a second workpiece. A fastening method includes enabling the threading of a first member of a fastener into a first workpiece; and enabling the threading of a second workpiece to a second member extending outwardly from the first member and the first workpiece.

A tool presses a joining element against a first object into a first opening of the first object while mechanical vibration energy is coupled into the first object and the joining element. Vibration energy and pressing force make the thermoplastic material of the distal portion of the joining element flowable and interpenetrates structures of the first object to yield, after re-solidification, an anchoring of the distal portion of the joining element in the first object. The second object is placed so that a proximal portion of the joining element is inserted into a second opening, and the first and second objects are pressed against each other while mechanical vibration energy is coupled into one of the first and second objects, until thermoplastic material of the joining element proximal portion is made flowable and interpenetrates structures of the second object to yield, after re-solidification, an anchoring in the second object.