N:N Relationships

I need to relate Assets to Opportunities in a N:N fashion.  I created the N:N relationship Asset:Opportunity and also set the Display option.  What I am not seeing is the ability to add the same Asset on another Opportunity - I get the error that the Asset is already related to another record.  

I do see that there is also a 1:N relationship Opportunity to Asset.  But I need an Opportunity to have N Assets, and those same Assets to have N Opportunities.  

Ideas?  Do I need to remove the 1:N?  I thought an N:N could be layered on top.  

Thanks in advance.