Nyquist Theorem adding two same frequency near to Nyquist Frequency with phase shift
When you add 2 or more sinusoids at the same frequency $f_o$ but with different phase shifts you get a sinusoid at same frequency but an additional attenuation term. Mathematically, you will have following:
$$cos(2\pi f_ot + \phi)+cos(2\pi f_ot + \theta) = 2cos(\frac{\phi - \theta}{2}).cos(\frac{2\pi f_ot + \phi + 2\pi f_ot + \theta}{2})$$$$= 2cos(\frac{\phi - \theta}{2}).cos(2\pi f_ot + \frac{\phi + \theta}{2}),$$You see basically, you have an attenuation term $2cos(\frac{\phi-\theta}{2})$ which will change the amplitude of the sum of the sinusoids, but the frequency of the sinusoid remains at $f_o$. Hence, if you were sampling it with $f_s&gt;2f_o$, then even after adding them you will not have any aliasing.

Only thing of concern is that because of attenuation term driving the amplitude of the sinusoidal, your signal might become 0 now, as in you example of $2^{nd}$ sinusoidal at the same frequency $f_o$ but having phase difference($\phi - \theta$) of $\pi$ compared to $1^{st}$ signal. That means $2cos(\frac{\pi}{2})$ which is equal to 0, and hence you have actually nothing to sample, both signals are completely out of phase and cancelling each other.