Due to the fact that I do not have a flash unit, and I would like to improve lighting conditions and photographic techniques, I thought maybe I can use the flashes of two cheap DSLR cameras as flashes and use my Nikon DSLR with its flash as a master. Do you think it is possible? I Hope I was clear enough. The cameras as flashes are D5000 and a Coolpix, and the main camera D610.
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.
improve lighting conditions and photographic techniques
Whatever you do is with those two objectives.
Try to get some Yongnuo flashes with a remote trigger. They are really cheap and works really well. There are some other "Cheap" but not cheap at all, like Godox.
I would rather search for some used flashes.
Sell the used camera and you can get some decent illumination.
Let me assume you don't want to sell the camera.
You could even just buy a used remote cable hot-shoe and used manual flashes. Even a continuous light lamp will be in my opinion a better option to improve your lighting technique.
Regarding your question, you probably would need to:
1) Work in a really dark environment.
2) Use a long exposure.
3) Ask a family member to trigger the other camera when they hear the click of the first camera.
4) And I say the click because the built-in flash will simply make the typical flat illumination.
You would certainly need to buy special triggers to synchronize the cameras and getting that to work right would almost certainly cost more than a cheap flash or some used equipment that is better suited to doing this right. The only way I could see you using a secondary cameras as off-camera flash would be in a very dark room, with a still subject (table-top product shots, for instance), and a very long shutter speed, during which you could do other things to affect the lighting, like triggering other cameras/flashes, or even just turn on and off a flashlight. In my experience, if you don't really need strobe to freeze motion, continuous lighting can be better, anyway, so you might also want to look at inexpensive LED video lighting, etc.