Hey Zachary, you'll want a pair of good ¼" TRS cables to go between the FocusRite and the speakers, both the Scarlett and the Eris 4.5's have balanced jacks (which is nice). Of course you can spend a TON for Mogami or Monster cables (which I have), but these LyxPro and now finding their way into my studio and live acoustic rig, and they are very well made:
The TRS cables plug into ONE of the Eris 4.5's, then you simply need good ol' speaker wire to run to the other cabinet. Here's my suggestion:
Great wire, it's really flexible so it immediately lays flat when you unravel it. And it's a really cool two-tone blue to match the blue theme of the Eris design.