You technically could do a Y-valve but then you'd always have a little bit of the beers mixing which isn't ideal. And then you need the valve mounted somewhere externally unless you want to open the fridge to switch beers. Spend the extra bucks on the 2nd tap IMO.
You'll want to Y the gas lines or get a distributor so you can get gas to multiple kegs. Then figure out how to get the 2nd tap mounted. Either add a new tower or drill through the door.