I did not mention stomach. I meant the GI-tract as a whole.
I've used food coloring and indigestibles (like corn kernels) to do experiments on whether meals can "overtake" or "merge" or "join" with other meals into poops.
Food coloring is a liquid dye. It will mix with whatever chyme it encounters in the stomach and small intestine, dyeing a large portion of the stool. It does not prove that food stayed in a single-file line.
Also, again the GI-tract as a whole is also not a "one-lane road".
Please educate yourself and do not do "experiments" on yourself. A good place to start learning more would be: https://accessmedicine.mhmedical.com/book.aspx?bookid=691 if you're intrested.
that's the most insane thing i read today. kudos to your curiosity