So I did understand you right, and so my answer to:
" Do I only hear the left or right panning? "
Is the same; you will hear just one of the channels. Suppose you lose the right channel, all you will hear is just a mono signal of what's in the left channel. When the sound from the Canvas is hard panned to the right, there will be no sound at all. When it's panned to the left, it's the loudest.
You need to wrap your head around the fact that there really isn't such a thing as left or right in signals. Stereo RCA out is two cables carrying separate mono signals. One is the "left", one is the "right". Panning the Canvas sound is just changing the volume balance between the two mono signals. You're shoving 2 mono signals via one connector into 1 mono input on the Behringer, and so one of them gets cut out. Let me say that again.
You're shoving 2 mono signals via one connector (the 1/8" jack) into 1 mono input on the Behringer, and so one of them gets cut out.
TO CONTRAST, the line-in on your computer knows how to process both of those channels. It is different from the Behringer.
I'm not sure how to make this clearer at this point. There's only so much an explanation will do for you, you have to actually play with the technology to see it working. Would you like me to make a diagram for you? Would that help?'
P.S. Don't tell me how to use the forums. I don't post here just for you, I post for everyone.
Also, WHY are you using an audio interface and then just putting it into your computer's line-in anyway? That doesn't make any sense. That just brings the noise back. There's a USB cable on the back of the Behringer. You hook it up to your computer via USB and have FL Studio read the audio directly from the Behringer box. I think the reason for the confusion is that you're trying to record into this box and then direct monitor the Canvas sound straight towards the Behringer's outputs and then fire that out of the box into another cable into your computer's line in. It's unnecessary.
Eliminate the computer line-in entirely, that's what you need to do.