They do, I ran tests on this in one of Contragor’s topics a few months back on Guardians of Empyrion and a few for myself on Storm Totem, Wind Devil and Blade Spirit. DoTs from player-pet attacks stack but debuffs do not.
So on Wind Devil, Howling Winds would NOT stack as it is a debuff aura but multiple Maelstroms from multiple Devils would.
The easiest way to test this is with Blade Spirit as bizarrely, Whirling Blades is considered an attack, so having 2 Spirits next to a dummy puts out twice the Bleeding DoT as 1 Spirit and can double if you command them to attack with Blade Eruption on top.
You can see the difference on what is considered a debuff and attack by looking at the petdb:
The above wouldn’t stack.
The above would stack.
For other uses, this is also arguably why x1x1x1x2’s Dreeg Bros build can put out huge Poison ticks, as he adds an insane amount of DoT damage to each Guardian and stacks several of them. I also have theorised that Demonslayer and Blade Spirits can also put out sizable, passive Bleed ticks as well.
@Ziller Great build as well, looks all around solid 
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