Could an electric airliner be aerotowed to cruise altitude by an electric "tug" aircraft?
Yes, a tug would help, but not greatly so. If your goal is zero-carbon air travel, the tug will stretch the possible range, but the result will still be much too low for practical use.

The principle is similar to using a catapult to save the energy needed for acceleration on the ground. This topic was covered here and shows negligible gains. We also have covered the problems of fully-electric airliners and the weight disadvantage of electric engines, so I will not cover this here again.

If we only look at the energy needed to lift a conventional airliner to its cruise altitude, the magnitude of the problem should become obvious. Here, only the tug is electric while the airliner is a normal A320. We have lots of answers with A320 data, and assuming that less fuel is taken onboard since we get some help in climb, the take-off weight used here is 72 tons. Cruise is in 30.000 ft, so we need to lift 72.000 kg by 9144 m. Energy $E$ is mass times height times gravitational acceleration, or 1.8 MWh or 6,456 MJ. Add to that the acceleration to Mach 0.8 which is 240 m/s at altitude and we need $\frac{1}{2}\cdot m\cdot v^2$ = 2,074 MJ more.

The sum of 8,530 MJ is the energy content of 200 kg of jet fuel, but for a realistic value we need to divide that by the efficiency of the jet. The same has to be done for the electric propulsion case, but efficiency is much better here. If we assume an overall efficiency of 80% in case of fully electric propulsion, the electric energy need to lift an A320 to cruise conditions is 10,662 MJ. To overcome drag and keep systems running, we need maybe another 25% more, so the total energy need will be 13,328 MJ.

Let's consider a tug which carries half its weight in batteries and has the same take-off weight as our A320. Its 36 tons of batteries will hold 25,200 MJ of energy which is just enough to bring both of them close to cruise conditions. On its way back it will glide, so we can save the fuel for lifting the A320 up. How much is that? If we assume an overall efficiency of 33% for jet propulsion, the savings are 600 kg or one twenty-fifth of overall fuel consumption over a 5000 km trip.

Even with the better efficiency of electric propulsion, a heroic effort is needed to save a comparatively small amount of jet fuel. It does work, but the gains are small compared to the effort needed - just consider that the turnaround time for the tug is similar to the flight time of our A320, so as many tugs as A320s are needed to make this system work for all flights.