Pierce potato with a fork in several places. Add 1 cup of water to the Instant Pot. Insert the trivet (the rack that came with the Instant Pot) and put whole potatoes on top of the trivet. Close the lid and turn the valve to a Sealing position. Press the "Pressure Cook" or "Manual" button and set the time to 20 minutes for medium potatoes (or 25 minutes for large potatoes).
When the potatoes are done cooking, quick release (QR) the steam. Drain the potatoes and remove from Instant Pot.
Once the potatoes are cool enough to handle, cut them in half lengthwise and scoop out the flesh using watermelon scooper or a spoon, leaving the skins intact.
Put the potato flesh in a large bowl, add butter, sour cream and milk and mash with a potato masher. If needed, add more milk and sour cream, until desired consistency is reached. Add 1/2 cup of shredded Cheddar cheese to the mashed potatoes and mix until incorporated. Add salt and pepper to taste.
Put the mashed potatoes into the potato skin shells.
Add 1 cup of water into Instant Pot and insert the trivet. Put the filled potato shells on the trivet and sprinkle with remaining 1/2 cup of shredded Cheddar cheese and then sprinkle with bacon crumbles. Close the lid and turn the valve to a Sealing position. Press the "Pressure Cook" or "Manual" button and set the time to 5 minutes on High pressure.
When it's done cooking, quick release (QR) the steam. Carefully remove the double baked potatoes from the Instant Pot, sprinkle with chopped green onion and serve.