Hi koen,
In China, this feature is very normal for most company, and we called it 'Counter Sign'. Unfortunately there is no nature support for this feature, so if you want to assign task to multiple users, you should do it by yourself.
For example, you could use CustomActivity and create a ExternalActivityBehavior to implement this feature, you can create multiple tasks for there users and assign a task for each of them, In jBPM 4, task could have sub tasks.