Customers and Contacts - how to upload data for many-to-many relationship

Hello,Normally we've been uploading customer and contacts with a 1-to-1 relationship and I'm exploring the option of many-to-many for customer to contacts. What I don't see is an easy way of uploading the data to support this.

How would I go about uploading a dataset that connects contact Joe to ABC Boats and XYZ Cars as well as XYZ Cars to Joe (the same) as well as Bob and Mary?

Thanks,
Peter