I don't think this can work, even with remote shutter controls, as the timing is critical:

• if you use a DSLR as second "flash", there's a delay to get the mirror out of the way before the flash fires. Your main camera is also a DSLR, and will have a similar delay. But, nothing guarantees that the delays are the same or even close. That means you have a good chance that the second flash fires before or after the shutter on your main camera is open.
• If you use a compact as a second "flash", you won't have the "mirror delay". But there are perhaps other delays... Basically, no synchronisation here either.

But even if that would work, build-in flash units are usually rather weak. As said in the comments, you are better off buying some external flash units, with an optical slave unit (the off-camera flash fires when it 'sees' the flash from the main unit). If you get Nikon units, or third party flashes made for Nikon, you might even have automatic exposure control.