[Forage]

Is it possible to configure dual audio as: HDMI and ALSA (for eg. Wireless audio) to work ?

No. xbmc has no native dual audio ability.
The Pi branch has the ability to drive two audio devices on the GPU side. The GPU has no knowledge of alsa, so this only works with HDMI and analogue.

There is a different dual audio patch that handles it on the arm side.
It is reported as working on the Pi but at a cost. It increases the arm cpu by about 25%.

What's the performance impact on enabling dual audio in it's current form?
I take it you'll loose the performance gain of using HDMI audio pass-through because the RPi has to process it for analogue output any way.
How is it compared to not using HDMI audio pass-through in the first place?

In my case I want movie audio through HDMI and music through analogue to prevent having to turn on my TV for music (DLNA control). If dual audio means a less movie decoding power than I'm not so sure I want to use it.