The collective locomotion of natural swimmers or flyers undergoing flapping motion is a ubiquitous phenomenon. Existing studies have demonstrated that collective locomotion usually can benefit hydrodynamic performance when compared to that of an isolated swimmer/flyer[1]. In those studies, flapping foils with prescribed transverse motion undergoing longitudinal locomotion are commonly considered [2]. This study adopts a different driving strategy, i.e., employing an external force varying sinusoidally with time at the leading edge of each foil, to enable the foils to move freely in both directions, thereby focusing on exploring the collective locomotion of two such foils. In particular, the influences of bending stiffness EI and phase difference ϕ are explored systematically using an in-house solver based on the immersed boundary-lattice Boltzmann method (IBLBM) and the finite element method (FEM). Preliminary results show that depending on these factors various self-organized collective behaviours can emerge spontaneously, including the staggered-following (SF) behaviour, semi-tandem (ST) behaviour, and side-by-side (SS) behaviour, as shown in Fig.1. The cruising speed of two foils is similar to that of the isolated one. Generally, when exhibiting the SF behaviours, the efficiency (η) of the foils is usually higher than those of the isolated counterpart, while it is not the case when other behaviours are present.