Hi Dorte!
You probably got an answer for this, but I will just reply in case somebody will need help with this in the future.
If you scroll below there are "Participants" where you can add people from different companies to this deal.
But creating a custom field for this also works. Depends on how clean look you are going for.
Sadly adding the person to 2 different companies is still not possible.

Best regards,
Andre