We propose the design method of fiber parametric wavelength converters based on dispersion-flattened photonic crystal fibers (PCFs) with two zero-dispersion wavelengths (ZDWs). Analytical expressions of the optimum signal frequency and maximal pump tuning range are deduced. By our method, the tuning ranges can be considerably broadened with a relatively low pump power and short fiber. This is because the fourth-order dispersion coefficient of two ZDW PCFs can be 1-2 orders of magnitude larger than those of one ZDW fiber and are effectively utilized to compensate the linear phase mismatch due to the second-order dispersion, resulting in a low phase mismatch for a widely tunable pump. To exemplify the effectiveness of our method, a PCF based on lead-silicate glasses with two ZDWs spaced 127-nm apart is presented. Numerical simulations show that based on this PCF a transparent wavelength conversion with a 117-nm pump tuning range can be achieved with only a 3.7-m-long fiber and 0.532 W pump power.