If you use the Qlik Sense .Net SDK you can connect to the same session and you will be able to do selections and also get change notifications to react on selections from other instances of the same session. Extended Current selection is a one way to do this. I recomend that you open the same object from the SDK and listen to changes and make selections on the object. Note that you can also do paging.
Here is an example that show much of what you need.