The unbelievable lightness of being a union employee in Wisconsin

The Walker administration has posted some truly extraordinary "perks" (booty) of being a union employee in Wisconsin. It's pretty incredible, breathtaking, actually, and has to be read to be believed. Here's a sample, although an amble on over there is well worth the trip:

Teachers Receiving Two Pensions

Due to a 1982 provision of their collective bargaining agreement,Milwaukee Public School teachers actually receive two pensions upon retirement instead of one.  The contribution to the second pension is equal to 4.2% of a teacher’s salary, with the school district making 100% of the contribution, just like they do for the first pension.  This extra benefit costs taxpayers more than $16 million per year.

Source: February 17, 2010 Press Release, Process of developing FY11 budget begins Milwaukee Public Schools

Explain, please? What is this "two pensions" of which you speak?