Does Signal allow multiple accounts or one account on multiple phones?
Signal can only be registered to one phone number per mobile device. If you have a dual SIM phone, you must choose one number to register with Signal.
How does Signal messaging work with dual SIM phones?
Just as if there was a single SIM, Signal communication is available with the Signal registered number.
- All Signal-to-Signal communication is private and end-to-end encrypted.
- Signal can use whichever internet connection is enabled on your phone, WiFi or mobile data from either SIM.
How do I use dual SIM for sending SMS/MMS?
Insecure SMS/MMS messages from your mobile SIM card numbers are sent through your mobile provider. Enable all permissions for Signal under your Android Settings for this to work. Long-press the insecure-send icon 
to choose which SIM card to use for the SMS/MMS message. Signal references the names of your SIM card as identified in your Android phone's settings.
To see dual SIM options:
- Update your OS to Android 5.1 or later.
- Go to your Android phone Settings > SIM cards > check the SIM labels.
- Make sure your SIM is enabled and has a mobile plan to send SMS/MMS. You may want to physically swap the SIM cards on your phone.
- Most devices: Go to Android phone Settings > Dual SIM settings or Dual SIM management > Messaging settings > check your settings are accurate. Samsung devices: Go to Android phone Settings > Sim card manager > Call settings > check that your Text Messages and Mobile data options are accurate.