I have power running to a 2 switch gangbox. Each

You will need the fallowing materials 12/2 wire, enough to go from the existing two to the top of the stairs were you are putting the new switch. You will need a single gang remodel box also called old work. One switch, and wire nuts. You will need a flat and philips screw driver and wire strippers/cutters.

First disconnect power to the existing switches.
Remove the two switches that are in the box
Locate the two black wires that go to the lights. one will be for one half and the other for the other half of the basement. these are your switch leg wires mark them so you will know what they are
Locate the other black wire from each switch. these are probably pig tailed into one wire in a red wire nut. the wire nut will have three black wires in it. You will disconect the two wires that went to the switches. Keep the pig tails you can use them later
Next you will find a wire nut with three white wires twisted together. remove the wire nut and untwist the wires.
You should have three bare copper wires. usually under a green wire nut. these should also be conected to each switch. again take these twisted wires apart.
Next you will run the new 12/2 wire from this box to the new single gang box. Cut the hole for the new box using the box to measure your hole. next bring the 12/2 wire into the new single gang box.
secure your box to the sheetrock.
Take the insulation off of the wire and expose the white black and bare copper wires.
Put the black wire on the bottom screw of the new switch and put the white wire on the top screw and finally the bare wire on the green screw. make sure your wire is secured well to the switch
Screw the switch to the box and install the cover to the switch.
Next go down the the two gang box here were are going to make the top switch take over these two.
Take the black that the two switches pig tailed into and put this and your new black wire together with a red wire nut. use new nuts
Next take the white wire from your new wire and connect it to the the bottom screw of each switch. you may use the old pig tails you took off of the old switches and be sure to use new nuts
Next connect the black wires that were at the top of the old switches back on the same way.
Next connect the ground wires all twisted together. you may need a larger wire nut our you can pig tail three and two.
Put both switches back in the box and install the cover plate
turn the electricity back on to the switch box and you will be able to walk up stairs with both lights on and turn them both off at the top but make sure you turn them back on when you reenter the basement or neither one will work below. Once you have turned the top switch back on though they will both work just like before.

hope this helps

As long as the power is running from the breaker panel to the switches rather than the fixtures then it should be pretty simple.

1. Run a new 2 conductor wire from the existing box to the new box at the top of the stair.

2. Install a switch at the top of the stairs.

3. Disconnect the hot wire that is running to the two existing switches and connect it to one of the two conductors running to the top of the stairs.

4. Connect the remaining conductor on the wire going to the top of the stairs to the two existing switches.

Note that if you want to be able to use the Dimmer function from the top of the stairs this will be much more complicated. You will need to put two new switches at the top of the stairs, run more wire, the new switches will need to be 3 way dimmers and the existing switches will almost certainly need to be replaced as well. The wiring will also be much more complicated.