For client to client you will not need a router.
Option 1 get two 10g NICs, two 10g SFP+ Modules, and a OM3 fiber cable
Option 2 get two 10g NIC and use Direct Attached Copper
Option 3 get two 10g NIC (RJ45) and use Cat6/Cat6a cable depending on the length. This might be the best option for future expansion.