"Ish" the fact that your amp is 2 Ohm stable means that you can use your existing subwoofer in a bridged mode (assuming that this subwoofer is 4 Ohm, not any less). I do not have the manufacturer's specs for the 6903 but all the Kemwood amps that I have seen and installed (I was working as a professional car audio installer until 4 years ago), bridge by connecting the Left (-) output to the (-) terminal of the speaker and the Right (+) output to the (+) terminal of the speaker. Again the other two outputs remain disconnected. O.K. Now as far as the input is concerned, most amps have a switch (sliding or press-in type) that must be configured accordingly for bridged operation. In other words, connecting the output cables as described above is only half of the bridging process. This switch is usually located in the input side of the amp usually next to the input RCA cables. Check one more time to see whether there is such a switch. Almost all the newer model Kenwoods have this switch. In the older amps this switch was sometimes missing and bridging the amp required either a special "bridging module" or a special "two into 1" cable that is available at Radio Shack. So if your amp does not have this switch, then you will need to get this cable. Keep us posted on what you found.
Again, it may be easier to contact Kenwood technical support (your local car stereo dealer should be able to give you their number), and they should be able to tell you how to proceed on this.