Yes, your neuron will follow the other neuron on governance proposals. This is because 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. "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 more than half for yes or at least half for no. If neurons in the list of followees abstain from voting, the majority may never be reached and the following neuron may abstain from voting.