Playing behind the same IP is fine, as long as only one of you only ever had one character doing any Harvesting/Excavation/Logging/Mining/Fishing (“HELMF”). As long as you guys are playing on characters bound to separate forum accounts, you can even play multiple characters each (a main and an alt out in the field as full-fledged characters, and a “mule” account that is restricted to staying in/around towns and capped at level ten).

Aside from preventing people from abusing HELMF with multiple simultaneous characters, the main concern with playing behind the same IP is that if one of the people behind an IP breaks any rules, buys gil, etc., punishment will likely come down on all the folks that use that IP.