If you have backed the T shirts with a fusible, I would think you could sew them to each other. Since you are using an overlock stitch they should be fine. I think most people use sashing because it helps stabilize the T shirt fabric edge. It will be extra soft with just the T shirts and will be loved when it's done.
One of my favourite T shirt examples on QB is under the title, "2 T-shirts quilts.....totally screwed up one". It was posted by watterside and if I ever do one, that is the pattern with the shadow look I want to do. There are many more great examples on QB just type in T shirt quilt in the search box. Good luck.