Anyone who has had any dealings with twins knows that finishing each others' sentences is simply par for the course - it's what most twins do.
In this instance, they are parroting set answers to questions - so knowing what words are coming is a matter of rote, nothing more than that.
Why Susannah Reid felt the need to slow her own speech down as though she was speaking to a five year old with English as a second language is not advised.
I research interviews with musicians, and if I ask a musician a question that has been asked many times before, I will get a set answer word-for-word - it's absolutely nothing to get excited about.
The notion that twins live together and are close is similarly hardly news - millions of twins do the same, so why these two are an 'internet sensation' speaks more to the population of the internet, than the notion that this is all new and dazzling, or indeed amusing in any way.