That's an interesting and unconventional idea. If I remember Lilium's [0][1] numbers correctly, most of the motor power is needed for vertical take-off and landing and only a small fraction (1/10 to 1/5) is needed when they have transitioned to wing-borne lift. This means that supporting the aircraft for just 1 minute during take-off would give 5-10 minutes of extra vertical flight, which translates to 25km-50km of extra range at a cruising speed of 300km/h.

[0] https://www.lilium.com [1] https://en.wikipedia.org/wiki/Lilium_Jet