It's quite easy, and technically [this is a duplicate question][1]...but since you framed it within the context of *lighting* I will go ahead and answer.

Say you have two or more lights, and you want them to share a value for emission.

For any property you want to share between multiple materials, you need to Group it. <kbd>Ctrl</kbd><kbd>G</kbd>

In this case the Value we want to share is to be used for Emission, so create a lamp with a Value input node for emission strength and then group that node.

[![Value node used to specify Emission strength][2]][2]

You should end up with a Group like this:

[![Group node for controlling Emission Strength][3]][3]

Hit <kbd>Tab</kbd> to go inside of the Group node and you will see your Value node that globally controls the emission strength for all lights with a material controlled by that Group node.

[![Emission Strength Value node][4]][4]

Does that make sense, or do you need me to elaborate on some points?

You can start by examining and playing with the example [.blend][5] provided.

The subject is lit with a proportionately controlled amount of light.

[![Suzanne lit by lights of different color, type, and distance, but the same emission strength][6]][6]

In this example render Suzanne is lit by multiple lamps. The distance, color, and even type of the lamps is different, but the global brightness is controlled by the same group-linked value.

Just keep in mind that if you're matching to an emitter mesh, the brightness will vary according to the surface area of the mesh. So you'll want to create a mesh light of matching size.

[<img src="https://blend-exchange.giantcowfilms.com/embedImage.png?bid=4493" />](https://blend-exchange.giantcowfilms.com/b/4493/)


  [1]: https://blender.stackexchange.com/questions/93659/
  [2]: https://i.sstatic.net/E8KaK.png
  [3]: https://i.sstatic.net/PUgDT.png
  [4]: https://i.sstatic.net/zCks8.png
  [5]: https://blend-exchange.giantcowfilms.com/b/4493/
  [6]: https://i.sstatic.net/EsOLI.png