Argentina and Chile are creating a joint peacekeeping battalion. These will be professional peacekeepers, as well as a unit with troops from both countries. That's also pretty rare. As the UN desperately seeks nations to contribute troops to peacekeeping operations, they are even more desperate for troops who have some peacekeeping training. Few nations are willing to establish units of professional peacekeepers, but that seems to be changing. The peacekeeping duty is not done as charity work, the UN pays, and that, in effect, is a subsidy to the armed forces of the nation contributing the troops. Given the growing demand for peacekeepers, this establishment of a joint peacekeeper battalion by Chile and Argentina makes sense. It also helps build better relations between the two countries, which almost went to war with each other in the late 1970s.
If more countries establish professional peacekeeping units, the UN will probably give preference to these units when it comes to peacekeeping jobs and assignments. This is where the long desired "peacekeepers army" will come from, not from a "UN Army."