Pour the water into the insert set in your Instant Pot. Set a pressure safe rack in the insert.
Stir the two cheeses, chipotle chile, green chiles (if using), cornstarch, garlic, onion powder and salt in a 2-quart, 7-inch across, round, high sided, pressure-safe baking dish. Pour the cream on top of the cheese mixture but do not stir in the cream.
Cover the baking dish tightly with foil. Set the dish on the rack or trivet and lock the lid onto the pot.
Set the pot for pressure cook or manual. Set level to HIGH. The valve must be closed. Set the time for 20 minutes with the "keep warm" setting off. Press START.
When the pot has finished cooking, use the quick release method to bring the pressure back to normal. Unlatch the lid and open the cooker. Use hot pads or silcone baking mitts to transfer the hot baking dish to a nearby wire rack. Uncover, stir in the tomatoes until smooth, and cool for a couple of minutes to blend the flavors. Serve warm with tortilla chips for dipping.