Solved

Boost on both cars

I've just watch my race live and on the last lap a rival team used boost on both cars for the entire lap.. looking back over the results he/she has been doing this for a while. how??

You need to be more specific.  

Are you asking how they can boost for the entire lap ( easy on some tracks if you have saved you boost)

Or do you mean how can you use boost on both cars at some time (they have activated the link - the icon that looks like. a chain next to boost so using boost on one car automatically affects both

You have a grey buttom side by side with the boost. When you click it you link both drivers and then you are able to boost both cars at the same time.