SSAS Tabular Cube partitioning - number of partitions
In SSAS 2014 Tabular partitions inside a table process serially. In SSAS 2016 Tabular partitions inside a table can process in parallel. So in your version partitioning won't speed up processing if you process the whole table. 

However partitioning is great for use in incremental processing. If you partition by year and only 2017 rows changed then you can just process the 2017 partition. Also to archive off data older than 10 years old you can drop the oldest partition. 

I don't know where the 2 partition rule you mentioned came from but don't believe it makes sense. If you have a reference for that assertion with some context then feel free to share.