I have water-bath-canned double layers before. My big canner has two racks, and I place my second rack on top of the first layer of jars, then the second layer of jars on the second rack so the second layer of jars isn't directly resting on the bottom layers' lids. You still need enough water to cover at least an inch over the topmost jars. I did this with jars that are only a couple inches high; you also need to take into account the height of your canner.

What does the owners manual say? If you don't find the answer there, call presto to get accurate information on their canner.

Couldn’t find a reference to BWB capacity the first time I looked in the manual. But now on a second look I see in the fine print is says not to double stack for BWB. Thanks for the reply’s.