Arduino ADK, other board, shield or what? Sorry for bothering.
I have a problem with my project and I can't fine someone who can help me yet.

I need the next configuration:
PC --------------USB cable-----------------MegaA---------------USB cable----------------MegaB

I need serial communication with USB cable in both boards.

I have now solved the ( PC --------USB cable------MegaA) part.

And the other part, I have done PC------------------USB cable------------------MegaB

But I need do that like the first schematic.
I don't have access to the MegaB because this is inside a machine, I can't access to its pines, I need connect to it by USB Cable.
Is possible do that with ARDUINO MEGA ADK in the MEGA A position? or I need other kind of board or add one shield (like USB host)?
I need send one thing from the pc to MegaA and Mega A send the same thing to MegaB.
Is it possible? because I can't find some way to specify that I need recieve the information from one serial port, and send it to another different serial port. 
I need help, is very important.
Excuse my english
Thanks
You would need a USB host shield to achieve what you want.

The Arduino by itself is not capable of connecting to both a PC and another Arduino through USB without the addition of extra hardware.  In this case the USB Host Shield as detailed on the Arduino website:

http://www.arduino.cc/en/Main/ArduinoUSBHostShield

In the list of supported devices is:


USB to serial converters: FTDI, PL-2303, ACM, as well as certain cell phones and GPS receivers. 


The magic word there is ACM - that (or more specifically CDC/ACM) is what the Arduino USB Serial interface is.