I don't think that substitution will work out as you wanted.
Nixmatalization changes the composition of the corn hull, the treatment weakens the tough outer shell and generally makes it very different from untreated corn. And the precooked flour is, well, already cooked - boiled before grinding to flour. Their texture, cooking time and taste will be different - especially if you're looking for specific or authentic textures or flavors from your recipe. They will also have different nutritional values, since making the nutrients in the corn more accessible is one of the effects of the nixmatalization. Corn starch is something else altogether, it is just the inner starchy parts of corn (no hull and no grain) - it can't replace the textural or flavor elements that come from breaking down the hull with lime or lye which makes nixmatalized corn distinct. Additionally, I sort of think it would make the dough thicker rather than thinner, as corn starch is a binder or thickener - so I wouldn't think it would work to make a flour that's too thick work like a thinner one anyway.
One article mentions that the precooked flour (henceforth called masarepa) cannot be made into a glutinous dough. Gluten makes dough stretchy, so I'm sort of assuming that the dough won't stretch to make the tortillas thin, it may crumble or smear instead. The fact that it is traditionally used to make smaller, thicker flat-breads and corn-breads instead of tortillas (and many sites say they cannot make tortillas with masarepa) suggests to me that there is a textural problem. Another article mentions that the taste will be different - masarepa is milder, and it also cooks up denser and grainier.
So, here are some suggestions. You might be able to make up something closer to a tortilla by mixing a little wheat flour into your masarepa - you need something like gluten formation to make it stretchy - but between the masarepa's milder flavor and the dilution with wheat flour, it will not taste quite like a corn tortilla. You could just go get some masa harina, and save your masarepa for other dishes.
You could just go ahead and make actual arepas - mix up a dough (possibly find a recipe to your liking), and it should shape into thick disks. It won't be like your corn tortillas, but it is an authentic dish and can probably serve a similar role in the meal - as a starch, as a taste contrast and buffer to other foods, as a vehicle to carry your main dish (arepas are sometimes stuffed - it would just be more like a sandwich). If you mix in the corn starch... I think you'd still get arepas, with a thick pasty dough and not a stretchy one, possibly a bit less corn taste (since corn starch is not strongly corn flavored).
If you really wanted to try making a thin crisp flat structure from the mesarepa, you might have decent luck making a very thin batter and treating it like a dhosa (pour a ladleful, spiral the batter out thin, and cook crisp). If the problem is that a dough made with masarepa won't stretch, using water as the medium to let it spread thin should work. It still won't be quite like corn tortillas, but it is as similar-ish as I think you can get.