Physical substance makes up everything in the material world, spiritual substance makes up everything in the spiritual world. When you combine the two substances you get soulish substance. It's a whole new substance created by the combination of the two. Separately they are distinct colors. But combine them and they create a whole new color - green.
Soulish substance is BOTH the physical and spiritual substance combined.
When you take a physical body and combine it with a man's spirit you get a living soul. More importantly when you combine the Holy Spirit with a man's body you get God's soul or what the Bible calls the Divine Nature. We can be a partaker, or sharer, of that Divine Nature through the promises. Unfortunately the two terms ARE used interchangeably in most, if not all, Christian circles today. And I believe if the Church understood this, they would have more revelation about what it means to develop as a Christian.