Since you have RealBand, it's easiest to to use to record your vocals. The same audio input that's working with BIAB will work in RealBand and you have many tracks available to add live recorded instruments or vocals to your song.
That said, you can do the same in BIAB if you want. It's not as visual and also a bit more tedious.

<< "As far as i understand, in BIab there is not option to record a second audio track." >>


This is a common misconception. Yes, there's only one visible audio track in the BIAB Mixer. But that one audio channel can be used over and over to record as many additional audio files as you want. Most people assume the only channel in the BIAB Mixer that can have recorded audio other than a BIAB RealTrack is the Audio Channel. No. Any of the eight BIAB Mixer Channels can have audio by simply converting any channel to an Artist Performance Track or by recording audio onto the Audio Channel and converting that audio to an Artist Performance Track and then moving the Artist Performance Track to any other available BIAB Mixer Track.

There's another method to record a second or more audio tracks in BIAB. It's called overdubbing and can be found in the Audio Recording Window when you select the Record Audio Button. This is easy to use.
Record your first track and save the take
Check the Overdub Underlying Audio Box and Record a second take. You're given the option to save or redo each take and you can record multiple passes to add your own vocal double, harmonies, Backing Vocals, new instruments, a solo - practically anything you want.

Attached Files (Click to download or enlarge) (Only available when you are logged in)
Overdub Box.JPG (46.37 KB, 165 downloads)
BIAB 2025:RB 2025, Latest builds: Dell Optiplex 7040 Desktop; Windows-10-64 bit, Intel Core i7-6700 3.4GHz CPU and 16 GB Ram Memory.