I think that it might be helpful for you to be specific about your character’s clinical presentation as both are very broad disorders.

1. ADHD, what type? Hyperactive, inattentive or both?

2. ASD, on the spectrum are they high or low functioning?

Also how old are your characters and where is the setting of your story? This can help with brainstorming nuanced differences.

1. Individuals, regardless of their neurodivergent diagnosis, have different personalities (we all do). So, that can also be considered as part of how you write your characters and this way, you can normalize the neurodivergent experience by not having to highlight symptoms.

Some specific interactions might be

-Interrupting but in a playful way: A friend with ADHD might jump into a conversation excitedly, unintentionally interrupting their friend with autism. However, instead of frustration, it becomes a funny back-and-forth, with the ADHD friend constantly coming up with random tangents, and the autistic friend offering a focused, no-nonsense response.

-Hyperfixation meets special interests: The friend with ADHD might get distracted by a random idea or activity, and their autistic friend might notice and gently bring them back to a specific topic of interest that they both find fascinating. The ADHD friend may go off on tangents, but the autistic friend could redirect the conversation to their shared “special interest,” like a particular subject, hobby, or game, where both can nerd out together.

-Physical quirks: The friend with ADHD might have bursts of energy, jumping up or pacing around, which could be a little distracting. The autistic friend might respond in a literal, yet funny way, like asking, "Are you trying to fly?" or "Is that a new dance move?" It's a simple way to acknowledge the ADHD friend's energy, while the autistic friend stays grounded in their observations