To clarify, you are asking how an agency can sell one tour operators products over anothers
you would need to get hold of the operator of your choice and ask for their trade contract / commission structure
If you are asking how a travel agents products are sold by a tour operator, usually this would not happen, it would be the agent selling the tour operators products, you could however, have both aprties working on a package together - each providing product for the package and both selling the package