Sometimes we recommend that people make redis-main.… and redis-cache.${PRIVATE_DOMAIN_NAME} point to the same server, e.g. for staging environments. For those cases, since the database numbers are hard-coded, we should avoid using the same number (2) for both REDIS_SESSION_URL and KOBOCAT_BROKER_URL.

I didn't see any other overlapping database numbers, but I also didn't search exhaustively. Enketo uses database number 0 and cannot be configured.