...AFTER I'VE OPENED the file in the first arduino and the second file in the second arduino,
Oh no! One Arduino with 2 SD. Each SD must be on a different SPI bus so that both can stay selected the whole time. You could use Two Arduinos but why? Also the current SPI library may need work to be able to use a hardware serial port instead of the regular SPI port.
how can i read and send at the same write and log in the other? i can use the ReadWrite commands but how to send and write once opened the files?
PS: is there a method to give to the file i'm creating a specic name?
Usually data to and from a file device like SD card is done through a buffer. You read to fill the buffer and you fill the buffer before you write it. You could open two SD device objects each with its own buffer then read a buffer into one, copy that to the other buffer and write that out. Or you could change the library code a bit to make them use the same buffer and save the copy step.
I'm not sure but it might be possible to run both SD's at once using a circular buffer where one fills and the other reads until pointers catch up then one waits. The only thing more direct would be reading the byte from the input SPI port and writing it directly to the output SPI port or gate logic right across the wires.
Open a file that is not present, the name will be the name you gave it within the DOS 8.3 name limits.
The Arduino SD library has no file rename function and given that SD media has limited writes, it is probably a bad idea to want to change the names much. I would simply make the change to the card on my PC to do once.
Remember you can use a computer to prepare data on SD that the Arduino will use later.