1. The white light emitted by two different light sources can't interfere with each other because the light sources emit light through internal atomic transition, and the atomic emission is intermittent. Each emission forms a wave train with a finite length, and each atomic emission is independent of each other, and each wave train is irrelevant, without the same frequency, the same vibration direction and stable phase difference.

3. Add a constant phase difference or in phase, that's right.