Q:

Two FTDI devices

I have two devices which use FTDI chips to communicate with a computer.  Will it be possible to communicate between them using a USB cable? 
For one of the devices I have complete control over the firmware, for the other one I don’t.   What I am trying to achieve is communication between two embedded devices via UART but I only have access to the interfaces after the FTDI chips. 
Something I’m thinking of is generating some kind of code, so that I can send a message from my device (the one I can control the firmware) to make the other device believe that there is a computer on the other side. 
Any ideas? 

A:

I assume you are referring to FT232 or the like.  In that case your plan can't work, because those chips are USB slave devices, and two USB slaves can't talk to each other.  
To connect them, you'd need a USB host with two ports, the FTDI driver installed, and a small app that copies characters from one (virtual) serial port to the other.  Quite feasible with a Raspberry Pi.