How to have two voices play nicely together when they have the same rhythm. The steps below are assuming you are working "modally" or within a given scale. If playing outside of a scale, you have a few more options.
Like this melody for example: 
- Decide if the voice you are adding is above, below, or weaving around your other voice.
- lightly mark / sketch / pencil in the consonant intervals above, or below, or both of your given voice. The consonant intervals in two voice note-against-note are:
- Unison (same exact note):
(Hard to see here as they are the same note)
- Octave (same note different octave)
- Up:
- or Down:
- Up:
- Perfect 5th
- Up:
- or Down:
- Up:
- 3rd
- Up:
- or Down:
- Up:
- 6th
- Up:
- or Down:
- Up:
- Unison (same exact note):
- Decide how you want to start and end (if this is an isolated melody then having your start on the scales root (or in octaves with your other voice) and ending in octaves or thirds with your other voice is common.
- Then "play sudoku" and find your path through the penciled in notes from yourt starting note to your ending note, all while avoiding things that would make it awkward to sing
- Dissonant leaps
- Augmented second steps
- Leaps in the same direction
It's best to prepare leaps with a step in the opposite direction and recover with a step
Sometimes you can get away with leaping and bouncing back to the note you started from
- Parallel 5ths:
- Parallel Octaves:
- "direct" 5ths
- "direct" Octaves
- Once you have a path, write it out and play it. If it sounds good, great! If not, try again.