I do it fairly often with my closest gf and a lot with dp.
It may be saying the same thing or one of us says something as the other thinks it.
Take today, dp and I were driving through a city that we drive through often. We drove past a street that neither of us had been down and both started saying that we should try driving that way sometime. It's an old city that we go to often and both love. Neither of us have even walked down this street but it does come out where we usually go to park. We must've driven past this street hundreds of times and never thought to go down it never mind mention it.
Maybe it's a sign we're going to come into money and be able to move to this street? 
We'd need to get pots of it to be able to do that, not that I would want to be right in the city anyway.
It's a mystery but it does happen a lot.
Does anyone else experience this?