This paper presents a fluid-structure interaction (FSI) modeling and numerical simulations of coupled flutter of flexible filaments in a flow. Experimentally-observed "stretched-straight state" and "flapping state" of a single filament are numerically and mathematically analyzed to find analogies in FSI dynamics with experimentally-observed "in-phase" and "out-of-phase" flapping modes of two parallel filaments. Our numerical results show that the coupled flapping of parallel filaments is modeled as a single filament with different mass.