In your situation I would file both the amended returns at the same time and it would be best to include the additional tax. If you overpay they will refund the money.
If you just file one return it is possible they would reject the amended return(s) if the dependent shows up twice.
If they did reject the amended return then the IRS would send a clarifying letter and you would have to deal with another set of issues and a longer time frame.
