Arduino: FTDI to USB on another arduino
No, it is not possible to interconnect two USB slave devices, even with a hub.  You would need to have a USB host device upstream of the hub to shuffle data between them.  

There exist embedded USB host devices which could be made to do this, but programming them for one peripheral is tricky enough.  Therefore instead of using one to shuffle data between two slaves, it is probably more practical to use the embedded USB host to talk to only device, and use it's serial UART (pretty much all, except perhaps for highly specialized chips would have them) in place of the USB serial cable, as this will make the overall software simpler than trying to juggle two slaves downstream of a hub.  An exception would be if the embedded hub is really a miniaturized full featured computer with a conventional operating system and USB host stack; ie, on something like a raspberry pi you would just want to write software to shuffle data between the peripherals, possibly even (depending on the slaves and drivers) using serial APIs rather than USB APIs.

Another option, if both USB slave devices are ultimately USB-serial converters, would be to disable that functionally and directly connect their serial UARTS with a crossover cable (and any voltage translation such as 3.3v - 5v that might be necessary).