user10756: 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

user4143: 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.

10756: <p>Sorry for bothering.
I have a problem with my project and I can't fine someone who can help me yet.</p>

<p>I need the next configuration:
PC --------------USB cable-----------------MegaA---------------USB cable----------------MegaB</p>

<p>I need serial communication with USB cable in both boards.</p>

<p>I have now solved the ( PC --------USB cable------MegaA) part.</p>

<p>And the other part, I have done PC------------------USB cable------------------MegaB</p>

<p>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</p>
 4143: <p>You would need a USB host shield to achieve what you want.</p>

<p>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:</p>

<p><a href="http://www.arduino.cc/en/Main/ArduinoUSBHostShield" rel="nofollow">http://www.arduino.cc/en/Main/ArduinoUSBHostShield</a></p>

<p>In the list of supported devices is:</p>

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

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