You don't hook them to each other. You hook them both up to the same source. How you do that depends. If the source is stereo then a splitter as suggested above will work. You won't be better sound but a bit more volume and maybe spread if you position the satellites apart more. You may actually get less bass depending on the position of each sub and where you sit.
If your source has front and rear outputs then you could hook up the 2nd 2.1 as rear speakers. The woofer won't do much but you will get surround sound