What happens if an harmonic in a band-limited signal at Nyquist frequency is added to a 90 degrees out-of-phase replica of it? I'm interested about aliasing in digital audio and I wonder if aliasing can be produced by simple mixing of band-limited signals.
As I know a band limited signal can contain frequencies up to sample rate/2 frequency, the so-called Nyquist frequency. Is this right?
If that is so, what will happen if an harmonic in a band-limited signal at Nyquist frequency is added to a 90 degrees out-of-phase replica of it?
Will this create an harmonic with double frequency and could such an addition create aliasing?
No.  Adding two waves at the same frequency results in another wave of the same frequency.  Depending on the phases, there can be anywhere from complete constructive interference to maximum destructive interference.  If the amplitudes are the same there can be complete cancellation.  This applies to the Nyquist frequency too.

See the math from my answer to FFT Frequency bin relationship.

Hope this helps.

Ced