We study the phase separation that a particle-conserving (“canonical”) full-layered metal surface must undergo at temperatures between preroughening and roughening. The separation is into two disordered flat (DOF) domains of opposite order parameter with a step between them, each domain exhibiting a half-filled top layer. It is shown that both Gibbs-ensemble simulation and canonical Monte Carlo plus finite-size scaling, carried out on a specific lattice Hamiltonian model, demonstrate this phase-separation phenomenon microscopically. A number of existing particle-conserving molecular-dynamics simulations for fcc(110) metal surfaces are then analyzed, and it is found that some display clear, previously unnoticed evidence of this DOF phase separation. Its main signal is a plateau of layer occupancies with temperature, around values close to 3/4 for the first surface layer, and around 1/4 for the adatom layer. It is proposed that this unusual type of phase separation could be observable on sufficiently step-free metal surfaces.