Offizieller Kommentar
When you're dealing with 2 DJs and that hardware, the ideal configuration would be for both DJs to use rekordbox in EXPORT mode, with all hardware linked via ethernet. Note that you could also use the USB-ethernet bridge to connect to the ProDJ Link via USB connection instead of ethernet.
Using rekordbox DJ for back-to-back would be difficult due to the USB connections for the CDJs to achieve HID control, and only having one CDJ connected to each computer would be less than ideal, even if you could connect both computers to the DJM for audio output. My recommendation around this would be for one DJ to export his music to a USB drive and connect it to the computer of the other DJ, then you can both use one computer but have access to both your own libraries.