Microcontroller with two USB interfaces and high speed (480Mbps)
This is a rehash of USB host to host connector with PIC18F2550 isn't it? 

http://www.atmel.com/products/microcontrollers/arm/sam3u.aspx is an example of an ARM device with high-speed USB. Note that it only has one uplink port; I don't expect you'll be able to find a device with two uplinks. It will also be quite difficult to transmit data to another one at that speed.

I strongly reccomend that you downgrade your expectations to 12Mbps, at which point it gets a lot easier. Have you decided what sort of USB device you want to present to the host PCs? Are you prepared to write your own drivers or are you going to emulate something?