Hi Fernando,
it is not possible to be done.
Why? - because of human nature.
Ususaly one guy take the project and know real financial condition. They ussusly do not fit to other developers. And if for one project they can combine their efforts in other project everyone will think that he/she is undervalued and have to receive larger payment.
I try to do something like that when I had more work with my ex-employees and in the end I have to finish and clean the project inhouse and this disappointed me.

Usually the best way is - You could act as project manger and hire some one for particular task with exact requirements and cost. When he/she do the job, you forget for subcontractor - till next job.