Vladamir, just confirming (and I believe you are correct, though I have not used it):
The Nikon N80/F80 can support TTL flash using both the pop up flash and an SB-50DX attached to the body by the hot shoe. To use both, you must slightly tilt the head up (SB50DX) to clear the pop up flash. this would have the benefit of increasing light ouput.
Again, I haven't tried this as I do not have an SB50DX (and my N80 no longer functions.)