I have checked if there any similar topic in the Qlik Sense Community. I have found this topic that date from March 2017 : https://community.qlik.com/message/1230496#1230496
In this topic, nobody seems to have found a way to synchronise the selection for two differents application in the mashup. I am still looking for API service, an overlayer, a HTTP connection that will allow me to have such functionnality.
Do you know at the moment if there are a way to use the same selection for two differents application in the mashup, please ?