Phase Locking an NCO

If I understand the question right, you want to derive a phase shifted second signal from the original NCO signal rather than phase locking the NCO. At least you didn't mention which signal the NCO should be locked to. Phase locking of a NCO would basically work with a PLL, but apparently doesn't apply here.

To generate derived phase shifted signal, you have two options:

- use a dual output (quadrature) NCO and generate the phase shifted output by linear operations

- use a multi channel NCO that has independent phase and frequency settings for each channel