This means that a subcontractor can demand payment for services rendered by him even if the primary contractor is unable to collect from his own client. In a casual partnership, however, one in which one partner might hold the primary contract, the distinction between partner and subcontractor becomes fainter. It exists nonetheless, and primarily has to do with the manner in which the partners work together. They treat each other as professional equals, for example, and will consult with each other to solve problems. The hierarchy in the relationship is more or less flat.
Nobody gives or receives orders.
The partners share tasks. Furthermore, both partners work on the entire project, not merely on separate parts. That is, although they may divide tasks, neither considers the project complete until both partners' tasks are done. Finally, and most concretely, both partners are stuck if the client doesn't pay. Together they rise, together they fall.