Technically speaking Stefan, unless you have a multiple server license or a web farm license, your setup is not in compliance with Kentico licensing (unless you're doing an active/passive setup). If you're using a web farm setup, you can specify which server (by name) it runs on in the actual task configuration.
Since each server has their own running IIS instance, they may execute at any time. For instance server 1 runs a task now and in 5 minutes the other server is scheduled to look and see if it's time to run that task. If the caching is different because of restart times in IIS, it could cause one task to run more or less frequently than desired.