Wikipedia says a Kopis was a 90-cm single-edged forwardly curved blade (think Kukri, Falcata) and says a Xiphos is a straight, double-edged sword of about 50-60 cm? Could you clarify what it is you find so similar about them?
It's definitely an interesting topic. I personally know very little about Greek swords even though I really like the subject. The spear is more commonly associated with the hoplite, so I suppose the swords weren't given much attention in games and such. (Rome Total War and Age of Mythology, I'm looking at you!)