Android does not allow you to open input (ie recording) audio streams to more than one audio device at the same time. This use case is not supported.
Perhaps if you explain why you want to record from different built-in microphones at the same time I could suggest a workaround.
Update: The term Audio Device refers to a device capable or receiving or sending audio. An audio device can have multiple microphones and/or speakers attached to it, and these are represented as different channels.