Inertial particle microfluidics is an emerging technology for microfluidic particle separation and manipulation. We investigate the formation and stability of pairs of soft particles under mild inertia through immersed-boundary-lattice-Boltzmann-finite-element simulations. The behaviour of the pair strongly depends on the lateral position of the pair in the channel which in turn is softness-dependent. Our results demonstrate that particle softness must be considered in the design of inertial microfluidic devices for manipulating inter-particle spacing.