  When a pair of parallel buckling beams of unequal width make lateral contact
under increasing compression, eventually either the thin or the thick beam will
snap, leading to collective motion of the beam pair. Using experiments and FEM
simulations, we find that the distance $D$ between the beams selects which beam
snaps first, and that the critical distance $D^*$ scales linear with the
combined width of the two beams. To understand this behavior, we show that the
collective motion of the beams is governed by a pitchfork bifurcation that
occurs at strains just below snapping. Specifically, we use a model of two
coupled Bellini trusses to find a closed form expression for the location of
this pitchfork bifurcation that captures the linear scaling of $D^*$ with beam
width. Our work uncovers a novel elastic instability that combines buckling,
snapping and contact nonlinearities. This instability underlies the packing of
parallel confined beams, and can be leveraged in advanced metamaterials.
