This has happened under wartime conditions.
Pardo's Push Captain Bob Pardo pushed another F4 Phantom that had lost its engines into Laotian airspace after a raid on N Vietnam, to where the pilots could eject into less than hostile ground. All while Pardo's plane lost one engine due to battle damage and a fire.
Robbie Reisner pushed an F86 Sabre that had run out of fuel, out of N Korean airspace and over the ocean. Unfortunately, the pilot drowned when he ejected, landed in the ocean, and became entangled in his chute lines.
However... If you undertake a study of high profile commercial aviation incidents, you will find that almost none of them could have been mitigated with this scheme. Here's a quick spot check...
AF447 High altitude stall in the middle of the Atlantic. Went down in less than ten minutes.
JAL 123 loss of horizontal stabilizer... aircraft uncontrollable. Would have just taken the tow plane down.
USAir 427 malfunctioning rudder control unit on landing approach - no time, aircraft uncontrollable.
ValuJet 592 Fire in the cargo hold. Burned through control lines and probably incapacitated the crew.
BA 9 Lost engines due to volcanic ash. Three engines restarted. "I trust you are not in too much distress"
USAir 1549 Lost engines due to bird collision shortly after takeoff. Not enough time to do anything other than ditch.
Alaska Air 261 Vertical stabilizer jackscrew malfunctioned due to poor maintenance. Aircraft uncontrollable.
TACA Air 110 Lost engines due to storm. Landed on levee without damage by unbelievably talented pilot.
Lauda Air 004 Thrust reverser deployed inflight. Aircraft out of control. We learned that the certification that the aircraft could fly with a thrust reverser deployed was inadequate from this incident.
In all of these, only TACA 110 or BA 9 might have benefited from a tow plane, and even so, there probably wasn't enough time, as both planes could remain airborne for maybe 20-30 minutes.
So it's an interesting idea, but as of today, an airliner losing all engines is extremely rare, as contemporary gas turbines are very reliable - enough to certify crossing major oceans with only two engines. The funds for modifications for a tow hookup, the training in how to do it, and the maintenance of dedicated tow aircraft, would be better spent addressing the issues that are causing most accidents.