I got the impression that you are hardwired from your pistol to something that you wear on your wrist.
I also got the impression that you wanted to use the same display setup for both guns.
The wired setup is just blah!
Here is what I would do. Upgrade the wrist thingy to a GLCD. You can get a 128 x 160 for about $8 via adafruit. On this display, have all the info for both guns all the time.
Inside each gun you use bluetooth or simple RF. If you used RF, I would strongly urge the Anaren TI CC1101 modules they are only about 16mm x 6mm with a chip antenna. However, one of the big advantages of Xbee is that they can transmit data without a uController. If you use the Anaren, I am almost certain you would need something like an ATTiny to keep it small. For such really short range, the chip antenna in either case is going to be more than adequate assuming you mount the RF module in a plastic part of the gun.
So each gun transmits its own info when you change mags, shoot, etc... and it is updated on the screen. It's all wireless so no blah wires trailing from the gun(s) to the wrist.
I would be careful about getting excited about updating a visual site picture via an arduino. It will never be fast enough to make it useful.
And last, I have looked everywhere for a hack of one of those spygear video watches and everything I have found links to software hacks for the 'game' itself.