When the levers are set manually to the same position, the engines depending on their aging will not develop the same nominal thrust, some differences will appear on the indicated EPRs or on the indicated N1s, so no point to have them linked together.
With modern aircrafts, for instance the B777, even when flying manual thrust, in flight, when the levers are set just close to each other, an automatic « trimming » function is automatically activated, it is transparent to the pilots and it corrects the engines thrusts as similar as possible by sending corrective orders directly to the FADECs, but however also preventing the beat noise that might occurs with fans having exactly the same speed; this occurs without any motion to the levers.
Taxiing with a single engine out of two is frequent for fuel consumption consideration and for noise abatement in many airports.
While forced to wait in a long chain for your turn to take off, and advancing step by step, pilots do use frequently one engine out of two.
On modern aircraft, you do not fly manual thrust but with autothrust or with autothrottles
On AIRBUS, the throttles positions have notches, so that when you select takeoff, the notch will set the throttles at similar position, nevertheless the commanded thrust do not correspond to the EXACT throttles positions but to a computed FADEC order.
On AIRBUS, at thrust reduction altitude the throttles are set to position CLIMB, for all the remaining flight, the levers remain there, and the thrust is managed by the autothrust
