IIRC the first one was to pull the belt tight and get the pawl/extractor thingy to get hold of the first cartridge, the second was to chamber said cartridge while pawl/extractor thingy grabbed hold of the next cartridge. Or words to that effect.
"Loading was accomplished by inserting the pull tab on the ammunition belt from the left side of the gun - either metal links or metal tab on cloth belts - until the feeding pawl at the entrance of the feed way engaged the first round in the belt and held it in place. The cocking handle was then pulled back with the right hand, palm facing up (to protect the thumb from injury if the weapon fired unexpectedly, which could happen if the barrel was very hot), and then released. This advanced the first round of the belt in front of the bolt for the extractor/ejector on the bolt to grab the first cartridge. The cocking handle was then pulled and released a second time. This caused the extractor to remove the first cartridge from the belt and chamber it (load it into the barrel ready to fire). As the bolt slid forward into battery, the extractor engaged the next round on the now-advanced belt resting in the feedway, preparing to draw it from the belt in the next firing cycle. ..." Wikipedia M1919 Browning machine gun