With one engine inop on a 3- or 4-engined aircraft, you've lost 33% or 25% (respectively) of your engines, and you have 1 or 2 (respectively) left in case a second engine fails. (It doesn't happen often, but every once in awhile, it does. Eastern had a L1011 ferrying MEX
on two engines some years ago, and they lost a second one just after takeoff. They were able to horse the thing back around for a landing on that one remaining engine, no small feat considering MEX
's high sea level elevation and the heat.)
If one were to theoretically ferry a twin with one engine out, you've lost 50% of your engines. Irrespective of whether it would actually be able to get off the ground or not, you'd have zero redundancy. Should the remaining engine fail, you're going down, and quickly. Maintenance ferries are done without passengers onboard, but FAA regs also protect innocent folks on the ground.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.