TL;DR

Beeper doesn't fully support this feature yet. For sending messages, Beeper will respect the phone number you have set up as default on your phone settings. Beeper will manage all chats imported with the secondary phone number, as well as any received messages to the secondary number, while keeping the identity of the secondary phone number intact.

Q: We don't currently support Dual SIM on Android. Will Beeper always use the default number on Android Settings?

A: Yes, right now, it will use the default number if you start a new chat, but it also uses the number on which you received the message if the chat already exists. So, if their default is ABC and they receive a new chat to XYZ, the reply will go from XYZ.