Neither Briar nor Simplex support using multiple clients with the same account, which is too bad.
Briar: "Censorship-resistant peer-to-peer messaging that bypasses centralized servers. Connect via Bluetooth, Wi-Fi or Tor, with privacy built-in." https://briarproject.org/
Would be cool to have bluetooth & wifi sync on native Nostr apps. Would be useful in offline situations and countries where the internet is censored.