I have been using this method for three years or more, starting off with four bags and then cutting back to three, with between 1.5kg and 2kg per bag. If mixing grain, I split the different varieties equally between the bags. These are what we call laundry bags in Australia (designed to hold delicate items in washing machines).
Initially I used to tie the bags with twine, then moved to zip ties, before eventually sourcing some bags with built in zip closures which are excellent.
Care needs to be taken when adding bags to strike water in order to avoid any dough balls. My method is to add one bag at a time, using a paint stirrer to turn each bag over thoroughly to ensure that all grain comes into contact with the water.
There has been no need for me to reveal my method to fellow AG brewers, and they have given many of my beers the "thumbs up", with none of them querying my procedures. Most of these beers are in the 5%-6% ABV range.
It beats the hell out of lifting one bag, especially when I have nowhere, and I mean nowhere, to install a pulley lifting system. As a matter of fact, I would not be doing BIAB if it were not for the multiple smaller bag method.