Here is one technique in a 5-part series on how to finish seams.
Serged Seams (Two Ways)
For a split serged seam, start by finishing the raw edge of both fabric pieces separately. You can use a serger or a sewing machine. With the sewing machine, use a zig-zag stitch or one of the overcast stitches.
Place the fabrics together with right sides together and stitch with the seam allowance called for in your project/pattern.
Press the seam allowances open.
Alternatively, you can stitch the seam line before finishing the raw edges. Then serge the two layers of the seam allowance together. Press to one side. This method is faster, but does create bulk on one side of the seam.
The split serged seam is used when you want to keep seam bulk to a minimum or if the seam will need to be open in one area; for example, for a hidden pocket, a side slit, or a zipper.
The double layer serged seam is fast and works well for side seams of a blouse, a pant leg, and so much more.