Line x-ray emission from short-pulse laser-produced plasmas is studied via one- and two-dimensional hydrodynamics models coupled with planar atomic physics post-processor. Laser energy conversion efficiency to He-α line emission is shown to be significantly enhanced when laser prepulse precedes the main pulse at an optimum pulse separation. The calculated optimum pulse separation of 1.5-2 ns as well as emitted energy and spectra compare well with the experiment.