This is a great Use Case but unfortunately, we do not support multiple Predecessors/Successors which is your road block. You can currently specify two tasks to start at the same time using Link and configuring with STS (Start To Start). Then the secondary Predecessor/Successor would be configured with FTF (Finish to Finish).
Since multiple Predecessors/Successors are not currently supported, you have two options....
1 - I can submit this as a Change Request for a future release.
2 - You can post this in the Customization Forum and/or join one of our Advanced Q&A Webinars to work with one of our Customer Success Managers to see if this can be done via Customization (i.e. Workflow or Custom Action). This may or may not be possible with Customization
3 - Other options is to speak with an Account Manager regarding Professional Services.
Let me know how you wish to proceed.