Can I connect two pairs of HIGHWIRE back-to-back to reach 600m ?
Yes, you can. Note that there can also be up to 100m between the HIGHWIREs in the middle of the connections. If the coax cable is RG6 or RG 11, then they will deliver 400m and 500m connections respectively, and 100m at either end, before and after the HIGHWIRE devices. If the cable can be selected for the installation, then consider the LONGSPAN option, which cann provide the same 100BaseTX network connections up to 820m on Cat6 network cable.