Partly that's because plenty of significant events only happen to one of them. Partly because the contrast between them will let me frame their shared scenes in the most effective voice. The third "partly" is because Heathric and Adal are both "young" characters and their voices are still settling in. They need to differentiate themselves. Heathric's very sensitive to the people around him. Adal's a more action-oriented guy. This ought to be visible in how they narrate.
How can you tell one character's voice from another?
|Photo reference for Adal, despite his|
objections. (Zac Efron)
Other characters might notice what people are wearing, the decorations in the room, or be looking for a quiet corner to hide in. (That would be me.)
Level of detail: People pay more attention to the things that matter to them. Conversely, they'll spend less time on things that don't matter, or have difficulty addressing them. Some people are also more prone to long, complicated thoughts while others keep it terse and to-the-point.
Word choices: One's upbringing, personality, and formal training will also influence the vocabulary one uses. Since Heathric and Adal are both from low-tech, rural, agriculture-and-animal-husbandry cultures, their world is framed in terms of natural phenomena, animals, and a dash of superstition. In training, one is a shepherd and the other is a Ranger.
Add in their personalities and Heathric ends up with a vocabulary that leans towards animals, emotions, and a certain gentleness/acceptance/working with what he's given. Adal takes a more proactive tone, focusing on utility and seeing things in terms of hunting, pursuit, and discipline.
All of these are deeply entwined with character development and world-building, as you can see. What would you add to this list?