Hi @SimonP1,
This can be achieved with association labels. These labels are a way to name a relationship between records, in your case contact and deal. A deal could have the label “Primary deal”. Once set up, these labels can be used in workflows and you can decide which deal should be updated, based on the label of the deal (versus all associated deals).
Another way to approach this is to re-think your workflow as a deal-based workflow instead of a contact-based workflow. This sometimes requires assisting properties. So instead of a contact-based workflow that updates all associated deals, try to re-think your workflow as a deal-based workflow that enrolls deals where associated contacts fulfill certain criteria. (You can reference contact properties in deal enrollment triggers.) If you’re not sure how exactly to tackle this, feel free to share screenshots of your workflows.
Hope this helps!