feat: Allow initiating multiple sessions for a message

test/omemo_test.dart

@@ -15,7 +15,9 @@ void main() {
     final aliceMessage = await aliceSession.encryptToJid(
       bobJid,
       messagePlaintext,
-      newSession: await bobSession.device.toBundle(),
+      newSessions: [
+        await bobSession.device.toBundle(),
+      ],
     );
     expect(aliceMessage.encryptedKeys.length, 1);
 
@@ -32,7 +34,63 @@ void main() {
     expect(messagePlaintext, bobMessage);
 
     // Bob responds to Alice
-    final bobResponseText = 'Oh, hello Alice!';
+    const bobResponseText = 'Oh, hello Alice!';
+    final bobResponseMessage = await bobSession.encryptToJid(
+      aliceJid,
+      bobResponseText,
+    );
+
+    // Bob sends the message to Alice
+    // ...
+
+    // Alice decrypts it
+    final aliceReceivedMessage = await aliceSession.decryptMessage(
+      bobResponseMessage.ciphertext,
+      bobJid,
+      bobSession.device.id,
+      bobResponseMessage.encryptedKeys,
+    );
+    expect(bobResponseText, aliceReceivedMessage);
+  });
+
+  test('Test using OMEMO sessions with only two devices for the receiver', () async {
+    const aliceJid = 'alice@server.example';
+    const bobJid = 'bob@other.server.example';
+      
+    // Alice and Bob generate their sessions
+    final aliceSession = await OmemoSessionManager.generateNewIdentity(opkAmount: 1);
+    final bobSession = await OmemoSessionManager.generateNewIdentity(opkAmount: 1);
+    // Bob's other device
+    final bobSession2 = await OmemoSessionManager.generateNewIdentity(opkAmount: 1);
+
+    // Alice encrypts a message for Bob
+    const messagePlaintext = 'Hello Bob!';
+    final aliceMessage = await aliceSession.encryptToJid(
+      bobJid,
+      messagePlaintext,
+      newSessions: [
+        await bobSession.device.toBundle(),
+        await bobSession2.device.toBundle(),
+      ],
+    );
+    expect(aliceMessage.encryptedKeys.length, 2);
+    expect(aliceMessage.encryptedKeys[0].kex, true);
+    expect(aliceMessage.encryptedKeys[1].kex, true);
+
+    // Alice sends the message to Bob
+    // ...
+
+    // Bob decrypts it
+    final bobMessage = await bobSession.decryptMessage(
+      aliceMessage.ciphertext,
+      aliceJid,
+      aliceSession.device.id,
+      aliceMessage.encryptedKeys,
+    );
+    expect(messagePlaintext, bobMessage);
+
+    // Bob responds to Alice
+    const bobResponseText = 'Oh, hello Alice!';
     final bobResponseMessage = await bobSession.encryptToJid(
       aliceJid,
       bobResponseText,