Using Neurons to Vote on Proposals and Earn Rewards

If I have a neuron that follows the ICA on "all topics" and also follows another neuron on "governance," will my neuron ignore the "all topics" followee and follow the other neuron on governance proposals?

Per-topic configurations act as overrides.

For any proposal topic, a neuron can be configured to vote automatically by following the votes of a group of neurons, voting to adopt proposals whenever more than half of the followees vote to adopt, and voting to reject whenever at least half of the followees vote to reject.

"Default followees" are only used for topics where no followees are specified. So if you keep the ICA as default and add an override like ICDevs for a specific topic, your neuron will follow ICDevs on that topic.

If both the ICA and ICDevs are specified either as default or as a topic override, the majority will be followed. A majority is defined as followed:

If a list of followees is specified (either on the topic or the default), the majority (> 1/2 for yes or >= 1/2 for no) is followed. If neurons in the list of followees abstain from voting, the majority may never be reached and the following neuron may abstain from voting.

The vote of a neuron is never split, so whether if you vote yes or no, you'll receive the full voting reward.