Partitions are != network partitions. Processes on the same node can become partitioned, threads in the same process can become partitioned. Actors within threads can become partitioned from themselves or other actors.
In my experience network partitions are not the most common kind of partition although they are well represented, especially on some network topologies.
IMO you should never assume that any two things won't be partitioned as part of your expectation of correctness or availability.
Can you describe a partition of two actors within the same thread? I'd guess this would be in some sort of logic failure state, where either the consuming actor doesn't correctly receive its messages, or the producing actor doesn't correctly transmit.
In my experience network partitions are not the most common kind of partition although they are well represented, especially on some network topologies.
IMO you should never assume that any two things won't be partitioned as part of your expectation of correctness or availability.