outsourced workers - workers not belonging to the organisation who is hired to do the work. Let's say Joe runs a plumbing business, but has more work than he can handle. He then finds another company (his competitor) and outsources the job to them, while staying the middle-man, the link between the original contractor and the company actually doing the work.
Or, he could also be working with individual subcontractors (tradesmen working for themselves).
second contractor - not a subcontractor, it's a second person doing part of the work under the same arrangement as the first one. Generally a subcontractor doesn't get to deal with the customer directly, while a second contractor does.