I use Kontakt5, too. You have to load Kontakt instruments for each instrument slot in Biab separately. This is possible since Biab 2012, I think.
So, what I did in achieving what you want is:

Go to VST/DX Synths/Settings
(The Default Synth shows up - leave that one as it is!)
Click on the "Thru" button
Click on the upper "None" combo line under "Plugin" on the right hand side
Choose your Kontakt synth (the "16out" version!)
The Kontakt window shows up.
Load your instrument within Kontakt.

For me that was all. Should be the same for you. The other instruments load with the default synth.
I have some songs with up to 3 Kontakt instruments loaded. But you need a rather fast system for this. The Kontakt instruments need a lot of the computer's resources. That's why they sound so good.

I hope this can help you.

Ulrich