In pro audio, a stereo signal is 2 channels. You are going to want to connect the 2 jacks labeled main going into jacks 1 and 2 on the itwo. Do not engage the instrument button on the itwo. I am assuming that the AKAI is outputting line level, but just in case have the gain on the itwo all the way down on both channels, and then adjust up as needed. Using the line in should bypass the gain stage of the itwo, but just being safe.

Then in Studio one, you can either set up a new stereo channel, and select input 1 and 2 as the input. Or you can set up 2 mono channels, and select inputs 1 and 2 respectively for each channel, then hard pan in opposite directions. Its ultimately the same result. I prefer 2 separate mono channels, which will give you more control over placement in the mix when you get top that point.