2 answers
looks like there is no easy way to accomplish this. Vote for the below Idea. It may be available by this year.
https://success.salesforce.com/ideaView?id=08730000000DpQGAA0
I see that on adding Opportunity team trigger on Opportunity gets triggred. So you can put a query on Opportunity Trigger for the Object "OpportunityContactRole" (where contact role is stored). and check if there are more than one contact role for that opportunity. Though not a neat solution still it should work.
Regards,
Amit.