In regards to Online Check-In I will need to see what the rules are for the new check-in application when it comes to multiple passengers and eBoarding passes.
As far as the iPhone goes, there is a way to get multiple eBoarding passes on a single phone. To do this you will need to check in one passenger at a time. As soon as you attempt to check in more than one passenger at a time you will not get a mobile boarding pass.
Danny, eCommerce / delta.com | Delta Social Ambassador
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