Line a baking tray with a brown paper bag or paper towels and set aside. Preheat oven to 250ºF. Peel all the potatoes, rinse and pat dry. Get out 2 mixing bowls, one medium and one large. Using the large-hole side of a box grater, grate Russet potatoes into the medium bowl. Add enough cold water to the bowl to cover them by an inch or so of water. Grate the sweet potatoes into the large bowl.
When done grating the Russet potatoes, remove from the water with your hands and squeeze them out over the bowl of water. Place the potatoes on a clean kitchen towel. Wring towel with the potatoes over the bowl to remove as much water as possible. Empty the towel into the bowl with the sweet potatoes. Let the potato water sit for about 10 minutes so that the starch accumulates at the bottom.
Pour off the water, being careful not to pour out the starch at the bottom. Scrape the starch into the bowl with the grated potatoes. (Alternatively, you can use about 1/4 cup of potato starch purchased from the store.) Add scallions, egg replacer, matzoh meal, salt and pepper. Gently mix together with your hands or a wooden spoon. Let the batter rest for about 10 minutes so that the matzoh meal binds everything together.
Place a 12-inch nonstick pan over medium-high heat and add about 1/2 cup of oil. Heat oil until a pinch of latke batter dropped in begins to sizzle immediately. Using a 2-inch portion scoop (or a large spoon), scoop out batter and carefully drop into the hot oil. Gently flatten with a spatula to achieve latkes that are about 4 inches in diameter and about 1/2 inch thick. You should be able to cook about three at a time in a 12-inch pan.
Cook for about 4 to 5 minutes on each side, flipping over once when the edges appear brown and crispy. Transfer latkes to the lined baking tray to drain. Continue to shape and cook the remaining batter. Place drained latkes on a serving plate and keep warm in the oven. Serve hot with the topping of your choice.