Double-decking is the practice of running a canner with two layers of jars in it, one on top and one on the bottom, with a platform of some sort in between.
Some people call it “double-stacking.”
You just need some kind of ventilated metal platform between the two layers. Don’t stack jars directly on top of each other.
The purpose of the platform is to stabilize the top row of jars, so that they don’t tilt over or fall over on their sides. That would interfere with the sealing, and possibly allow the jars to bang about inside the pot.
The purpose of the platform being “ventilated”, as in having holes or perforations or spaces in it, is to allow boiling hot water, or steam, to move freely throughout the pot.
Double-decking for water-bath canning
Whether double-decking is endorsed or not for water bath canning depends on who you ask.
The National Center for Home Food Preservation says that double-decking is fine for both water-bath canning and pressure canning:
Can two layers of jars be processed in a canner at one time? Yes, two layers can be processed at one time, in either the boiling water bath or pressure canner. Place a small wire rack between the layers so water or steam will circulate around each jar. Make certain that the water covers the top of the jars by 1 inch [ 2 ½ cm] in a boiling water bath canner. The pressure canner should have 2 to 3 inches [ 5 to 7 ½ cm] of water in the bottom.”  NCHFP. Canning FAQ #8. Accessed January 2015.
The University of Missouri Extension says yes as well:
Q. Can I process two layers of jars in a canner at one time? A. Yes, you can process two layers at one time, in either the boiling water bath or pressure canner. Place a small wire rack between the layers so water or steam will circulate around each jar. Make sure the water covers the tops of all jars by 1 inch in a boiling water bath canner. The pressure canner should have 2–3 inches of water in the bottom.” Home Canning: Questions and Answers. Mississippi State University Extension. Publication 993 (POD-01-16). 2016. Page 5. Accessed August 2017 at https://extension.msstate.edu/sites/default/files/publications/publications/p0993_0.pdf
The University of Wisconsin Extension Service, however, recommends against double-decking for water-bath canning:
“We don’t recommend stacking jars in a boiling water canner because the water movement may tip the jars.”  Barb Ingham. Professor & Extension Food Safety Specialist, University of Wisconsin-Madison. Email to Randal Oulton. 9 February 2015. Email on file.
Presto, the canner manufacturer, does not endorse double-decking for water-bath canning. The Presto 23 quart manual (2013ish edition) says on page 11: “For boiling water method, do not double deck jars.” They don’t say whether their concern is (a) weight with all that water and jars (on the glass-top stoves that they certify their 23 quart canner for), or, (b) whether the concern is that double-stacked with ½ litre (US pint) jars there would be insufficient height left in the pot to cover the top layer of jars with water and allow room for bubbling water movement.
HealthyCanning.com tested the capacity of the Presto 23 quart (with cold tap water and empty jars half-litre / US pint with lids but without actually canning), and weight consideration aside, if it is standard height half-litre (US pint) mason jars that you are stacking, then no, the Presto 23 quart pot is not tall enough for water bathing double-decked jars of that standard height profile size without the water boiling over constantly onto the stove — and that is one of the tallest pots that the average home canner is likely to have, anyway. So, it’s a non-starter.
Double-decking in water-bathing does not affect processing time
Double-decking for water-bathing would affect the amount of water needed, clearly. But it would not affect the processing time. You still need to follow the recommended processing time for your tested recipe: no more, no less.
Double-decking for pressure canning
All authorities endorse the practice for pressure canning. It’s considered a safe practice, and even safe, cautious canners routinely do it. There is no issue with it as far as food product safety or quality goes.
Presto says, “It is recommended that you stagger the jars by placing one jar on top of two. Jars may touch…. Although it is not necessary to use a rack between layers of jars, if you with to do so, a rack can be ordered…..”  Presto Pressure Canner and Cooker, 23 quart model, #72-719F. 2014. p 18.
The reason you need to stagger the jars is to distribute the weight. You don’t want too much weight in one spot preventing air from escaping out of the lids in jars below during processing.
For pressure canning, there are many different unofficial double-decking and even triple-decking configurations that people do in Presto canners. See the separate entry on Presto canners for those.
Double-decking in pressure-canning does not affect processing time
Double-decking for pressure canning does not affect the water required or the processing time. You still put in the amount of water recommended by the canner maker — no more, no less. And, you still need to follow the recommended processing time for your tested recipe, no more, no less.
Double-decking for steam canning: No
It is physically possible to stack some low-profile jars in the Victorio multi-canner steam canner in a stable and secure manner, using a perforated divider plate from a pressure canner.
But, is it safe in processing terms to do so? In August 2019. we wrote to Wisconsin Extension, who did the steam canning research, to ask if there were a research-based answer.
Dr Barb Ingham replied that how effective the steam flow would be hasn’t been studied, so they are not recommending stacking.Dr Barb Ingham to Healthy Canning. Email. 16 August 2019. Email on file.
|↑1||NCHFP. Canning FAQ #8. Accessed January 2015.|
|↑2||Home Canning: Questions and Answers. Mississippi State University Extension. Publication 993 (POD-01-16). 2016. Page 5. Accessed August 2017 at https://extension.msstate.edu/sites/default/files/publications/publications/p0993_0.pdf|
|↑3||Barb Ingham. Professor & Extension Food Safety Specialist, University of Wisconsin-Madison. Email to Randal Oulton. 9 February 2015. Email on file.|
|↑4||Presto Pressure Canner and Cooker, 23 quart model, #72-719F. 2014. p 18.|
|↑5||Dr Barb Ingham to Healthy Canning. Email. 16 August 2019. Email on file.|