Interference fringe width is given by
${\mathit{\beta }}_2=\mathit{\lambda }\frac{\mathit{D}}{\mathit{d}}$
So, $\mathit{\beta }\propto \frac{1}{\mathit{d}}$
(i) If the coherent sources are infinitely close then the fringe width will be so large that a single fringe will occupy the whole screen. So, no interference pattern will be detected.

(ii) If the coherent sources are far apart, then $\mathit{d}$ is very large. So, the fringe width will be so small that those cannot be resolved separately. So, no interference pattern will be detected.