If you have access to a sweatshop in Indo-China you will have a better way of doing this in one pass with a double needle straight sewer and a dedicated folder but short of having this access you can do a reasonable simulation of a double stitched jeans seam using a 3 or 4 thread overlocker and a normal straight sewer. You need a couple of different feet as is shown in the photos below, one is a side fence foot often used by folks who are into quilting, the other is an open toed foot where you can use one side of the open toe as the guide for the face seam closest to the layed over edge.
The seam type is a modified flat felled seam with the underside edge secured by an overlocked seam. You can use either a 3 or a 4 thread overlocker, the 4 thread being a bit stronger.
This is the initial overlocked seam with the denim layed face to face like normal. Do yourself a favour and iron this seam as flat as you can get it using an ironing cloth so you don't fry the poly cotton overlocker thread. you do this to improve your accuracy using the side fence foot in the next operation.
With the overlocked edge as flat as you can get it, the next operation is done with the side fence foot to provide the line of stitching that determines the width of your double stitch face seam.
This is the first line of face stitching using the side fence foot adjusted to the required with and is stitched directly through the overlocked seam on the underside. In this photo I did the seam and then placed it back under the foot to show how it was done. you are using the edge of the layed over seam as a guide so that you get a reasonably straight face stitch. It is a really good idea to iron the layed over seam BEFORE you do the face stitching so that you more accurately control the line of stitching with the side fence foot.
On my particular machine I have used an open toed foot so I could use the inside edge as a guide for the second line of face stitching and have offset the needle to get the width from the layed over edge that I wanted. Different machines have different techniques for using an inside foot guide and each person will know what works best for their machine.
This is what the back of the finished seam look like, it has 3 lines of stitching which will make it strong enough and importantly for grades of denim that are prone to fraying before it is edges are secured, Note that I was a slob and did not wind a bobbin in the same colour as the face stitching so the back of the straight sewn seams are black but you should be able to see the 3 seams from the back.
This is what the finished outer appearance looks like. I fished this roll of cotton out of my junk and to get the stitching quality up to scratch the thread and bobbin tensions should have been adjusted to tighten the thread up some but as I have my straight sewer set up for Rasant, I was not willing to change it for a simple demo.
A seam of this type is fundamentally strong, it is flatter than a double rolled jeans seam which for many styles of garment will improve the appearance but while the technique is a bit fiddly, it shows that you can do this type of appearance without having to have access to a sweat shop in Indo-China. Things like denim skirts, jeans and jackets are the target for denim and if your patterns are good enough you can make things like this that don't fit like a sack of spuds like at least some of the Indo-Chinese ready to wear stuff does.
Just to appeal to your sense of humour, the denim was a Faustian bargain that I picked up for $5.00 a metre but found that in one direction (the light coloured threads) you could tear it apart so it only use is for ironing cloths and the occasional demo.