studio will not allow creation of many-to-many relationship

I have two simple custom modules created some time ago: Resellers and Sales Territory Management (STM)

I now have a requirement to relate one or more Resellers to the STM records and need a many-many relationship as each reseller can be assigned to multiple STMs and each STM can have multiple resellers.

In studio it will only allow the Sales Territory Management to be on the "One" side, regardless of whether I try to add the relationship from the Reseller end or from the STM end.

I have never encountered this before, what could be stopping the many-to-many?

thanks,

FrancescaS