Hi @torysher,
Which subscription level are you on? With an Enterprise subscription, you have the tools for blocking a deal from moving on if the number of associated companies is greater than 1.
Let me know and I’ll happily outline the solution, involving a custom view-only property and required deal stage properties.
Without a Professional subscription, it would only show up as a friendly reminder but could be bypassed. Let me know if you’re still interested.
Best regards!