A pc can only be the master in the USB. You cannot connect two hosts, but only hosts to slave devices. So you need to build a slave device that connects to two hosts and echo the traffic from the one "channel" to the other. Although that would be quite easy using a microcontroller, I believe that the driver would be quite painfull
Just a note, such devices are available starting at very low prices (30-40euros)
Every mcu can act like a usb device as long as it can interface a USB controller. The problem will be with the max throughput of the data sent. The usb controller will keep the received packet of data and will NACK following packets, until your slow f84 running at 100KHz reads the already received data from the controller. No data loss, just slow data rates.