I do not believe this is possible unless you created some sort of Customization (Custom Action) that would enable you to do this. Out of the box, you cannot update similar tasks (2 unique tasks with unique IDs) at the same time.
You do show logic in your explanation that could be used in a Customized Workflow rule...
" project A and project B using a hotel in Amsterdam on the same day" ...
Since there are 2 unique Tasks (one in each Project) for Amsterdam Hotel, you could create a Workflow or Custom Action that would allow you to populate the data in the two tasks with one entry/click.
This will require additional configurations such as workflow rules/custom actions to automate this process.
Let me suggest the following options:
We will be happy to build it for you. For that we would need to understand the requirement better to provide with a professional services quote for this. The best approach is to contact your account executive (Nick Seaver) to request a quote from our professional services team for this.
Alternatively, you can also join our free interactive Office Hours - Advanced Configuration Webinars ( http://www.clarizen.com/live-demo.html#second). These are great resources for customization consulting with our experts and visual answers to your questions. One of our Customer Success Managers will be able to demo some examples for you and give you guidelines how to build it yourself.