Stereo tie clip mics have the pickups (condenser inserts) pointing 180 degrees away from each other. The ones that mount in parallel to each other, like what we see on the news etc are, i think, just backups and mixed mono. It may help the spread pickup pattern too making it wider if the person turns their head to the side.
Using 2 lavs, especially one pointing down was a trick to reduce ambient noise in a location. Since the voice of the person wearing them was close to the mic, it had very little effect, but ambient noise further away would often cancel arriving out-of-phase at the two mic heads. Try it - it works.
Thanks for the info. So was there anything special about what Bob Costas was wearing? It looked to me like it actually curved the mics in such a way as to reduce rubbing against the clothing. Also the way the wires were mounted on the clip looked neat, almost like there was one wire that split into two from the end of the clip. Most likely it was just both wires threaded through the same entry point or something. Just never worked with one like that before and wondered it it was a specific kind of clip/mic combo.
Pointing a lav down can reduce plosives, (breath wind-pops) but can only be done with an omni directional lav, It may look a little strange, but does not change the sound much. Can't do it with a cardioid lavs though, but they should only be used where feedback from a PA system is an issue.
Using two lav mics are for redundancy, common for live TV newscasts and such.
They are NOT for stereo, as many would surmise.
Double micing for safety/backup is actually quite common for live national shows. There is no other sonic reason to do it.
When I was A1 for a remote broadcast of Bill Maher's show in DC, we double miced the host and all guests and brought both channels into the console, with only one fader up. If the one mic failed, we would have brought up the fader for the other.
This happens with both wireless and wired mics, and the cabling and cabling path are generally the same for both.....either under jackets or under shirts, and around to the back where there is either a transmitter or the cable goes out to an XLR connector and cable. Little pieces of gaffer's tape are used to hold the cabling to clothing.
When you are trying to hide the mic, things can get much more arcane.
Directional mics are almost never used, which means that the up or down orientation is not such a big deal. Placement is pretty important, though, and you can definitely hear someone turning their head away from the mic. The omni directional mics make bleed and "phasing" a big deal when you have a panel of guests, which makes really active mixing, gates and automix mixers a very common part of the process for the A1.