At what age do infants learn to bang two blocks together?

6 months. Infants generally start to grasp objects around 5 months of age, and often pass them back and forth between their hands around 5.5 months. Keep in mind that some kids are a little early and some kids are a little late. Plus or minus 1 month or so is still appropriate.
8 months. Unless a child is premature or has had some serious illness before this, most children will bang two objects together by around 8 months.