ANA IN 3/4 do not have to be connected to use ANA OUT 3/4.
But, you do have to know how to use the routing (which not everybody does – it's a bit difficult). My guess is that you are not feeding the outputs properly, so are not getting a result. Apologies if I'm wrong.
For example, if you are coming in stereo (as you say), why would you expect ANA OUT 3/4 to be driven ? They might, but this would depend on the preset and routing. What preset and routing are you using (name just one of each) ?.
One way to be sure the outputs are good would be to connect your signal to ANA IN 3/4, select routing (22 Analog A || B) and program (12 Thru). If you get a signal at ANA OUT 3/4, this will tell you something.
