Femtosecond laser-produced plasmas are bright ultrafast x-ray sources. Since the short x-ray pulses from such plasmas are synchronized to laser pulses, femtosecond laser-produced plasmas are important sources to probe the ultrafast responses of optically excited materials. Conversion efficiency has to be improved so that laser-produced plasma x-rays may be used in such applications. By using an independent ultrashort pulse, which has proper parameters, as a prepulse, one can improve x-ray emissions from laser-produced plasma. We report here the results of experimental and theoretical investigations of the line x-ray emissions from aluminum plasma created by a pair of femtosecond laser pulses in order to show the optimum energy conversion into selected x-ray lines.