The drain configuration is different when converting from a single to double bowl, especially with a disposer. Basically, the tail piece of the sink without the disposer goes to an EL then drins to an extension to a TEE where the disposer drain joins. The TEE leads to the P trap and drains into the wall. There are amost an infinite number of ways that these slip-joints, TEEs ELs and traps can be configured. Keep in mind that you are not limited to 90 degree angles, the centerline of the disposer and sink tail piece may not be anywhere near in-line. Some sinks are displaced up and back from each other. The point is, there are a lot of ways these sinks are constructed, and each one takes a slightly different combination of fittings to resolve.
It may take some experimentation to get the routing down, but remember that there are lots of parts that can be purchased to make almost any routing possible. I like to think of this as a two or three trip DIY job. It takes two or three trips to the plumbing supply store to complete it, but eventually it will come together.
Well, the trial and error approach has been my best shot, but you never know what better advise you will hear on this board. Good luck.