A biostimulator, such as a leadless cardiac pacemaker, including coaxial fixation elements to engage or electrically stimulate tissue, is described. The coaxial fixation elements include an outer fixation element extending along a longitudinal axis and an inner fixation element radially inward from the outer fixation element. One or more of the fixation elements are helical fixation elements that can be screwed into tissue. The outer fixation element has a distal tip that is distal to a distal tip of the inner fixation element, and an axial stiffness of the outer fixation element is lower than an axial stiffness of the inner fixation element. The relative stiffnesses are based on one or more of material or geometric characteristics of the respective fixation elements. Other embodiments are also described and claimed.