I have serious problems with deciding this now in BNW. In G&K it was a spot on river with access to one luxury and one bonus resources, both of which supplied immediate gains like marble and wheat. Now it's problematic mostly because of "luxury centers" - huge concentration of one luxury resource in a given area. Very often it's impossible to fund the 2nd city anywhere near your capital and still benefit from the above mentioned condition.
The bigger problem is something else, though. If my capital is not a coastal city, I usually strive to have my 2nd city located by the mouth of a river. Usually it's far away, quite often it's closer to AI than to my capital. It's not so easy, though. There's no gold either from rivers or from sea resources, therefore it's important to have at least 2-3 gold sources near your city. One must build his initial cities high in BNW, which means that they will have some infrastructure, especially religious, and they will also require defensive force. Barbarians are way more active in BNW.
To sum it up: the 2nd city need to be a trade hub, if the capital cannot play that role, and must provide some initial gold. The 3rd city is can usually be placed with border cohesion in mind. One bigger territory is easier to defend, linear infrastructure costs less and is easer to build and due to standard starting conditions, such location should be a good place to grow a decent city, even though most likely there's not going to be an additional luxury resource available.