Help! My husband is legally blind and we need more light in our bathroom. The only light source we have comes from two sconce lights above the vanity (controlled by one switch). We'd like to know how to convert these two wires to one bar light.
The difficulty that you face is the existence of two outlet boxes, the metal or plastic boxes recessed in the wall that contain the electrical wire. Most electrical codes require that those boxes remain accessible, with either a fixture, a device (such as an outlet or a switch), or even a blank cover over the outlet box.
If both of the sconces are controlled by the same switch, you only need to hook up the new horizontal bar to one of the sets of wires in one of the outlet boxes behind one sconce. The question is, where is the power coming from and how are the outlet boxes and the switch routed?
In the simplest example, the power comes to the switch box. The hot wire (black) is attached to the switch. The neutral wire (white) is not attached to the switch. On most modern switches, the bare ground wire (occasionally green) is attached. In the switch box, there is another set of black, white and bare wires. That second black wire is attached to the other pole of the switch. That is now a switched hot wire. The second white is attached to the first white and the ground is attached to the other ground(s). These wires then leave the switch box and go to the closest outlet box behind a sconce. The switched hot wire is attached to the black wire from the sconce and also to another black wire going to the second outlet box for the next sconce. The white wire is also attached to the white from the sconce and another white to the next sconce. Similarly the ground wire.
Several other wire connections are possible and a bit more complicated. If you are unfamiliar with what was just said, you probably should not be trying to do this without an electrician.
On the practical/decorative side, someone skilled in electrical work can easily hook up a bar light tapping into one of the outlet boxes. If the second box is directly tapped only off the first (as described above) and has no other wires in it, the wires leaving the first sconce to power the second sconce box can be fully disconnected, and the second box will no longer have service. It could then be fully covered without access, or even removed. The dead wires in each box should be marked to be sure there is no confusion in the future.
If the wiring does not allow you to fully remove service from one box, it either needs a blank cover over it, an outlet with a correct cover, or it needs to be fully covered by the fixture.