The easiest way to run two amps in stereo is to run your guitar signal through a stereo effect such as a Stereo Chorus. (See the attached graphic.) A stereo effect splits one signal into left and right parts and lets you run them into two amplifiers. The chorus is a particularly good choice for this job because it lets run one side of your stereo rig wet -- with some effect (in this case, chorus) on it, and the other dry with no effect. In stereo, the wet side produces great depth, and the dry side retains your definition. The combination of the two sounds positively huge. In fact, two 1x12 combos run in stereo sound bigger and louder than a full stack in mono even if they're not.

Notes:

• Generally, you should not perceive the wet side chorus as a "chorus effect" in the stereo rig. That is, you're just running the chorus to provide depth. You want hugeness -- not a noticible chorused sound. So in a stereo rig, run the chorus subtly.
• When you run two amps in stereo, you want to keep the speaker cabs roughly three feet apart to produce the optimum sound.