How do I turn two light fixtures into one?

The wiring is most likely run from the switch to the first fixture and then on to the second fixture. If this is the case, you can connect your new fixture to the first electrical box, assuming its location suits you. You could then simply place wire caps on the unused wires to the second fixture.

I don't recommend cutting or removing the old wires because you may want to return to the old layout someday. Also, if you have any problems with the project, it'll make it easy to undo the installation.

If you need to move the wires to suit the location of the new fixture, the proper way to do this is to install a new fixture box. Run wires from the old box and connect them with wire caps. By code you cannot patch over the old box, you have to make it accessible by putting a cover plate on it. Alternatively, you can run an unbroken length of wire from another earlier electrical box, which is probably the box that contains the switch. That way you can patch over both of the old fixture boxes.

The lights should be mounted on electrical boxes. Wire nuts put on the end of the wires and then placed in the box will be the correct way to do this. A blank cover is found at the local electrical store. It depends on the location of the electrical boxes just how you will connect the new fixture. You may need to open the drywall to get the wire where you need it.

Buy a new mirror/medicine cabinet. Make sure the new one will cover up the old junction boxes on either side of the old mirror. Run a new wire from one of the junction boxes to a new junction box for your new fixture. cap off the wires that ran to the old fixtures and cover the boxes with metal plates. Never leave caped wires in a wall without a junction box protecting them.

very simple. open the junction box that served as the tapping point of this connection, from there-you can condemn this extra line. or, you can hide the extra line inside the light that you want to install or at the back of your mirror, only dont forget to seal it w/ wire nut. also, you can open the switch cover, and detach the 2nd wire connection.

there are many ways to do this but it still depend on the actual site

situation.

Ripping the wall? consider it your last option. it'll only complicate a simple problem that requires a simple solution. rippings means you have to CHIP the concrete wall,then restore the plaster and the painting afterwards.

You do not have to put in a separate light. At home stores you can find bases that will receive light bulbs and serve as outlets too! Go for the best of both worlds. I have included a source for you to check out what I mean.

That is basically what you do, either tape them off or cap them. The other option is to see if they are daisey chained through the first j-box. disconnect the wires there if that is the case.