Update on Using x sync and hot shoe to fire multiple flashes. If all else fails, read the instruction manual.

Well it was worth a try but it didn't work. No matter how I try when I pop up the on camera flash my flash wired to the x sync terminal does not fire. It seems the pop up flash disables the flash sync terminal.
The quickest and easiest fix may be to buy "Digital" slaves. I have several peanut slaves that I use to fire remote flashes. When I fire the camera with a manual flash attached, the slaves sense the original flash and fire the flash it is attached to. Digjtal slaves do exactly the same thing but somehow they ignore the pre-flashes that TTL auto flashes emit to determine the correct flash exposure before they fire.

Using x sync and hot shoe to fire multiple flashes. If all else fails, read the instruction manual.

I have six flashes that I carry with me everyday in my camera bag. Some are made by different companies so they don't always work together too easily. I recently read Joe McNally's "Hot Shoe Diraries". It has got me excited about using small flashes in a new way. I have always been very much a STROBIST (http://www.strobist.blogspot.com/) kind of guy where I use all my flashes on manual and adjust accordingly. Joe's book (http://portfolio.joemcnally.com/#p=-1&a=0&at=0) encourages people to use the brains of Nikon's engineers and the "Creative Lighting System" of the Nikon wireless flashes. They work great! It's really amazing to throw a couple of flashes around the room, flip up the little pop-up-flash on the camera and control everything from the camera. The only thing is I have a couple of old dinosaur flashes that I use regularly too. I was photographing a renovated kitchen the other day with a big bright window in it so I needed every flash I had to raise the light level in the kitchen (really dark cabinets) to balance the scene through the window. I thought I could use the dinosaurs to give a general over all level by pointing them up to the ceiling and use the new techie Nikon flashes to tweak different spots. I placed my flashes (6) all over the room and started firing. I couldn't get these things to all fire! What's with that? I finally put everything on manual and slaved them and fired everything from a dinosaur mounted on the hot shoe. It worked but I was a little disapointed I couldn't use the new technology. I was trying to fire the Nikon Speedlights from my pop up flash set as commander. They worked perfectly but the older flashes would not fire from the wired x sync terminal (front left little socket on the front of the camera). I could not figure out why! I finally had a brainstorm. Read the manual. It took less than 5 minutes to figure it out. Page 384 of the Nikon D-700 manual (Flash Contacts) says, "The Sync Terminal. A sync cable can be connected to the sync terminal as required. Do not connect another flash unit via a sync cable when performing rear-curtain sync flash photography with a flash unit mounted on the camera accessory shoe." I presume that also means when using the pop up flash (in effect a mounted flash). I always have the rear shutter curtain flash set for when I need low light mix of ambient and flash.