Gear down ferry should be somewhere at the back of the AFM or DDG.
The section that has various corrections and penalties for bits missing etc. Sorry, I can't remember the name but CDL (configuration deviation limits?)comes to mind.
Most jet twins will fly OK at ferry weights with an engine out and the gear hanging out in the breeze. Most will not, and are not required to, fly at high commercial weights with BOTH an engine failure AND the gear stuck down. Because many certification requirements do not assume multiple failures, i.e both an engine quits at V1 and the gear won't come up. You may as well say both engines might quit therefore all airplanes have to have at least three engines and be capable of flying away from V1 on only one of the three.
On the other hand, if you do get to do a gear down ferry, you do have to hold the weight back to some pretty low value and probably the AFM will call for increased V1 just in case one engine fails, as that is the 1st (and only) assumed failure on that particular flight. En route performance and fuel also has to be adjusted.
But if a tacho fails on an ordinary flight and you can't get the gear up, you've had your one failure on that trip, as far as the law is concerned. Common sense says you would not dispatch if you discovered it before takeoff (or maybe before 80 knots if that is your SOP) and if it happened late in the takeoff or after takeoff you'd return for a landing, if possible "just in case". But if the departure airport wasn't suitable for a return you could still go somewhere else without breaking the law, even if performance was compromised and even if you gobbled up your fuel reserves.
This exercise makes an interesting LOFT and I am sure it has been done to death in the simulator. However, to subsequently introduce an engine failure at a point where the airplane can not arrive somewhere under full control is unreasonable. Multiple and simultaneous or near-simultaneous failures are sometimes taken too far by some simulator instructors who can spook ordinary god-fearing line pilots with unrealistic scenarios.