At a high level, there are a few concerns about the stability and supportability of a server with both Qlik Sense and QlikView installed:
Port Conflicts: Out of the box, both QVS inside of QlikView and the Engine inside of Qlik Sense use 4747. This can be changed in Qlik Sense, but not QlikView. Additionally, HTTPS in both products would encounter issues with the default usage of 443.
Resource usage: Both products are resource intensive. Under load each will want more CPU and RAM than may be available due to the consumption of the other. RAM can be handled by the working set limits in each product (e.g. setting the low and high to be below 50%). But this will degrade the experience of each product, assuming the total RAM isn't effectively doubled.
Installer Requirements: The Qlik Sense installer checks to see whether QlikView is installed and will terminate if found. Given this behavior, you can theoretically install Qlik Sense then QlikView and achieve a minimum level of stability. Future upgrades may run into this same blocker.
Unfortunately, there is no workaround to have QlikView or Qlik Sense installed in the same machine/server. Currently, it is best practice to have both software separated and individually installed on their own servers or machines.