1. The ONLY thing that matters is if it sounds good. Scales and such are just a guideline. There are no fast rules. If it sounds good, it is good. (Perception is reality.)
2. In re 2 notes, and speaking of scales, try specific intervals within a scales, 4ths, 5ths. In blues it is pretty common to play notes from adjacent strings, for example, if I am playing in the 5th position I might hit the A-D on the 7th fret (4th and 3rd strings).
If that isn't as clear as mud... LOL