How do friendly stealth planes locate each other?
The accounts that have been published of F-117 missions indicate that they generally flew single-ship, rather than formation missions, and it seems probable that B-2 aircraft do likewise.  So in a sense, the important question for those aircraft isn't "how do I know where (another stealth aircraft operating nearby) IS," but "how do I ensure that we don't collide with each other," and that's a much easier question.

There are several ways to "deconflict" multiple aircraft operating in relatively close proximity that don't involve seeing or painting each other on radar.  One aircraft operates at one altitude, another operates at a different altitude is one method, while timing over a point (you get to waypoint ABC no later than 19:00:00, and I'll be there no sooner than 19:00:30) is another, and physical separation (you stay west of this longitude line, I stay east) is another.  In practice, all of these can be combined to ensure that "your" mission is deconflicted from "my" mission.

Of note, it isn't only stealth aircraft that will use tactics like this.  Helicopters operating blacked out at night don't have air-to-air radar, and even with NVG's, the ability to see each other isn't reliable enough to plan on "see and avoid" and the "big sky theory" to preclude a mid-air, so they'll use some fairly detailed deconfliction plans incorporating separation in space and time.

More recent stealth aircraft such as the F-22 and F-35 are reported to have datalink capabilities so that they can "see" each other without radar, by means of an encrypted, stealthy broadcast of each aircraft's position and track.  This would allow multiple aircraft to fly in formation (not close formation, but perhaps 1000' or more apart), and to work with other aircraft equipped with the same datalink, so even though the bad guys would only be able to see the non-stealth aircraft on radar, the friendlies could all see each other.