Two PRT-10401 LiPo Chargers in Parallel
500mA is not enough to charge that battery in a timely manner (24.8Wh/(0.5A*4V)=12.4h), and that's if you have it connected to a device that it can negotiate with. Consider putting together a current-limited switching regulator with a power MOSFET that can push at least a couple of amperes to the battery (or, you know, just buying one).

But don't put multiple chargers in parallel since even small differences in output voltage could cause damage to either or both chargers.