I've done several engine out ferries in 727. Boeing provides takeoff data for one engine inop takeoffs and you compute runway and climb requirements to meet the same standards as when all engines are turning. Major part of flight crew training covered loss of a second engine during takeoff. Depending on the dash number engine and conditions, you're good up to 140,000 pounds. Had a sim default back to 170,000 pounds one time and got off the runway to 100 feet for about 10 miles while the sim instructor engaged some 'speed' dumping. As said above, when you get to 200 knots clean, you've got it made. We pulled the C/B on the ground interconnect to gang the electric "B" system to the engine driven "A" system hydraulics. Depending on whether you had engine hydraulics or not, the commit point for landing was flap extension or gear extension.