Sorry but your question may not be able to be answered unless you provide more information. Ideally it would be nice to know what you are trying to do and what resources you have already. Be as specific as you can and also tell us what you have accomplished to this point. Pictures and diagrams also will enable us to give you a better response.
The output of the sensors can be read by two inputs of the Arduino. If the sensor outputs an analog output (a range of voltage levels) you can use one of the Analog inputs and if the output of the sensor is a high or low voltage you can use one of the digital inputs. The integration of these two sensors is then done in the software that you write to make the decisions and control the output of the Arduino to control the light. In summary the multiplexing that you asked about is done by the software in this instance.
It is not a simple as that. You have to have a detailed understanding of your environment (where this will be used), the sensors, and output devices that are being used and a bunch of things that get dictated by the resources, parts and pieces that are available for the job. Once all these things are clear then the programming can begin. While I am happy to try to answer the questions that you may ask about specifics it isn't practical to do long distance coding on someone else's project. I would recommend that you check out some of the already completed projects that may be posted on the internet and see if one is close to what you want to do. You could then try to build one similar and with a little study you can modify the existing code to suit your needs.
Do you have a working prototype of your ultrasone sensor by itself, or the pir that you're using, or for the system that switches the light and controls the intensity?
Those projects could become the start of your future project.
It'll be easier if you have each of them working, and have experienced what happens.
