Confirm one harmonic map
Before arranging two instruments, agree on the performance key and verify the chords in a representative verse and chorus. Compare any disputed label with the recording together. If one player uses a capo or a transposing notation convention, distinguish the sounding chord from the shape name in your notes. Both musicians need to understand the harmony the audience hears. Use the synchronized lyrics to identify shared entrances, but do not assume matching screen positions automatically guarantee matching rhythm. Count one passage together with minimal playing before building separate instrumental parts.
Assign different jobs
Ask one player to carry the steady pulse while the other uses longer chords or sparse responses. For guitar and piano, the guitar might establish a light strum while the piano leaves space between chord attacks. For guitar and ukulele, try different rhythmic densities rather than making both parts busy. These are starting experiments, not fixed rules about instruments. Play the same verse twice with the roles reversed. Listen for the version that leaves the melody clear and lets each musician hear the other's timing without increasing volume to compete.
Coordinate changes in density
Choose one section where both players become fuller and another where one deliberately reduces activity. Agree on the lyric landmark that triggers each change. Rehearse the approach so the contrast sounds intentional rather than like someone has forgotten a part. If the groove becomes crowded, remove notes from one instrument before asking everyone to play more softly. Finish with a complete song and identify which part is essential if the other player drops out. Each musician should be able to maintain the basic form alone. Clear roles produce a flexible arrangement that can survive small mistakes without both players trying to fix the same space.
Try one pass where the second instrument enters only at the chorus. That simple restriction reveals whether the first player can carry the verse and whether the added part creates a meaningful change.