Can two sub-agents edit the same file at once?

No. Line prevents parallel agents from overwriting each other:

• Per-file locks: one agent edits a file at a time.
• Read before edit: an agent must read a file before editing it.
• Conflict check: an edit is refused if the file changed on disk since that agent last read it.

Edits to different files can run in parallel (when they don't need approval).