My GUESS is to just dye the center first with tape layed out for your pattern and wipe up the excess dye. Reverse for the out side and the lighter dye wont really show up over the darker. Is just a guess still learning Or you tube search it?
A bunch of info here - it's basically a multi-step process using sealers to block off the areas already dyed to prevent over run and bleeding - to keep it straight a steady hand and LOTS of practice (the new blue paint tape may help without leaving marks - try it with scraps.) https://www.google.com/search?sourc..........0.#q=coloring+two+tone+leather&spell=1
I use an airbrush. it won't give definitive lines between the two dyes but it will give a fadeing color from one into the other. I do that pretty often, I like the effect.