Originally Posted by
mrkemp1214
If I manage my booking with either, I see the full flight route, but because they're totally different airlines, with an 18 hour layover, I thought I'd be able to skip the first flight, and go LIS > JFK > LIS.
That's not how it works. You may have two different confirmation codes, but you only have a single e-ticket, and when you no-show the first leg, all downline legs will be canceled. Everything might look fine on the TAP system, but you either won't be able to check in or won't be able to board, depending upon when they try to validate that the ticket attached to the reservation is still valid.