The short answer is to connect all the same color wires together assuming the same color wires go to the same pins on each cable. It's worth beeping though to be sure that both cables are made the same way, though.
The longer answer is that red is normally +5V and black is ground so you don't want to swap those around. The green and white are data+ and data- (or vice versa, I'm too lazy to look it up) so it's a differential pair, which is a fancy way to use two wires to make one logical connection that's much more impervious to noise. So it's not like a RS232 connection where one is transmit and the other is receive and you have to swap them at one side. Data- is just the logical inverse of data+, so swapping them will make it not work.
However, I'm a bit confused about what this cable will do. USB cables have a host side and a device side. The host is a type 'A' connector and the device is a type 'B' or 'C' connector. If you make a cable with two 'A' type female connectors that means you're connecting two host side connectors together which means you'll have a device on either side of the connection. Wouldn't you want a female A to male A to make an extension for a single cable or a female B to female A to make a bridge/extension between two cables?