Seafarers: The Fog Island -- discovering hexes with the Road Building Development Card
There is a clear distinction between placing and building.

The road building card says you place them as though they are just built. The hex exploration is triggered by placement, not by building. So logically, you would explore the first road's adjacent hexes before you have to place the second road or ship.

How else could you build the second ship/road if you intend to cross the mist?