I'm talking about outputs which we use to charge our devices with. Would using USB male to two USB female cord be a bad idea?
Plugging multiple devices into one powerbank for charging may not work because the resistors which set the maximum current draw may conflict with each other. It (probably) won't hurt anything to try.
You could try a USB hub as an alternative, but it will waste some power on its own.
Normally is it a bad idea to use a splitter cable on real USB devices, as the data pins would conflict. If you are using this for power only devices, ones that do not connect the data pins, then yes, a male to two female cable would work. If you are connecting two phones, then no.