Unbound electrons can experience resonant scattering and transient trapping in a molecular potential. In molecular photoemission, these shape resonances manifest as peaks in the cross section with a width of several electron volts. They depend on the details of the molecular potential and the wavelength of the photoelectron. We report experimental results on photo double-ionization which show the phenomenon of a collective two-electron resonance which occurs when the de Broglie wavelength of the dielectron quasiparticle is similar to that of a single electron exhibiting a single-electron shape resonance.