It's quite easy, and technically this is a duplicate question...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. CtrlG
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.
You should end up with a Group like this:
Hit Tab to go inside of the Group node and you will see your Value node that universally controls the emission strength for all lights that use a material that uses that Group node as a controller.
So by adjusting the value of that node within the Group, you can control the brightness of all lights that utilize it.
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 provided.
The result is you can have a subject that is lit with a proportionately controlled amount of light.
In this example render Suzanne is lit by two different Lamps. The distance, color, and even type of the Lamps is different, but the universal brightness is controlled by the same group-linked value.
