:: Can you give us any references or citations to differentiate the two, and clarify what precisely you are talking about? I'm finding it difficult to find any reference to "straight-two" at all, except those referencing the Wikipedia. On the other hand, I can find plenty that use parallel twin for 180, 270 or 360 degree twins, and referred to it as "most commonly known". If you read the references I gave, you will also see it is widely used for automobiles, snowmobiles, ATV, jet skis and so on, and has been for decades. 

 Even if you cannot provide references, I'd genuinely like to know where and from when the term was used. (What I think you are suggesting is that parallel-twin is used only for 360 degree engines but that is just not so).