[LAU] Recording just input 3-4 in Audacity

[HamRadio]

Hi,
I'm beginning to use my shiny Edirol FA-66 with ffado, jack and
audacity on my Debian Squeeze.
After reading some guides and hints on the web it seems to work fine,
even though I'm still learning how to effectively use this setup.
So I'd like to ask someone here on the list:
how can I make a stereo recording in audacity with *only* input 3 and 4?
I'm only able to get these signals if I start a project containing 4
tracks, thus having also 1 and 2 which of course are silent.
Perhaps I have to route 3 and 4 in some way that they appear on 1 and 2?
And how?
Many thanks for answering.