USB Host to Host communications

We are an Single Board Computer (SBC) running embedded XP. We need to
establish a USB communication link with another vendor machine. They are a
USB Host and will only accept a USB client connection. We too are a USB
Host.

Once the connection is in place we will communicate between the two using a
predefined protocol (API).

What is the best way to accomplish this?

I've seen some of these USB Bridge cables which clain to allow you to
exchange files between PC's but I'm not sure this will work for us. We need
to be able to send/receive to/from the remote machine programmatically
(C++).

We don't want to have to add hardware to allow our SBC to be able to act as
a USB client.

Suggestions?

Thank you!

Mark