There are two antennas cables doing same function at iPhone 7 plus?

Sorry, but I did not understant the functions yet. Even one disconnected cable as you can see on updated pictures, 4G signal is working, WI-FI is working. I also have tested calling for a number and it is working.

Sure that makes sense! As you have two of each antenna's one on the top and one at the bottom.

Think of it this way you have two water taps and two hoses that connect to one hose (Y connector) You are watering your plants as you turned on both taps the water is coming out both. But, now if you shut one of the taps is the water still coming out? Yes! As the second tap is sending the water instead of both.

This is the same design used here one on the top and one at the bottom so if you block one antenna the other will still be available.

So this parts exist for improve signal or do you think it acts just as a secondary function, working if the first one does not works?

I think you need a way to visualize whats happing here. Lets imagine you're in a room with a stereo system with two speakers. Sitting down in the middle of the room you can hear the music from both speakers if you moved just in front of the left speaker all you really hear is from that speaker, if you move to the right speaker you just hear the right speaker. So in this simple example you get part of the of music when you bias your self in front of each of the speakers or if you are just in the middle you get both speakers in full stereo.

Your antenna's work in the same fashion!

Now lets push this a bit further. This time someone disconnected the left speaker so you only get the right speakers music. Unless you knew the music was in full stereo you wouldn't know.

So if one of the antenna's was not connected or blocked you wouldn't know!

Ok. Thank you!