I'm not sure you understand how this works.

When DL/UA will finance a new plane, they also gain that plane in their assets. Their total debt will increase, but it will only have a minor effect on their debt to asset ratio.
The problem AAG has is that even with all the new shiny planes, they still are way down in the drain with their shareholder equity.

Not to mention their free cashflow and profit margins are in a different ballpark than AAG, so their ability to pay down any debt is a lot better than AAG, who is barely staying afloat right now.