I have a group of merged shapes. How can I make a double stroke in Photoshop?
I'm using Photoshop CC
The easiest way to do this is to add a stroke effect to the shape. In general, create your shape and give it a fill and stroke, then add a stroke effect via the Layers panel.
The stroke effect can be aligned outside or inside a shape, and Photoshop conveniently defines "inside" and "outside" to include any vector stroke already applied to the object, so a 5 pixel stroke effect can add to a 10 px vector stroke (outside), subtract from it (inside), or both (center) according to how you set the parameters.
Since the vector stroke and the stroke effect can both be solid colors, gradients or patterns, and a stroke effect can have any Blend Mode and Opacity you want, the possible combinations are mind-boggling (although I estimate that better than 90% of them pose a serious risk of eye-bleed).
As of the June 15, 2015 update you can add multiple strokes to things in Photoshop CC.
Simply open the layer style dialog on whatever you want to stroke and then click the plus icon on the right side. This will add an additional stroke to the shape.
Every stroke has its own set of controls so you can adjust any of the options individually.
Note: You can go up to 10 strokes per layer.
From Adobe's What's New page: