It is worth mentioning that the KU Leuven research group led by Moritz Diehl did an incredible amount of work on rotational startup of dual drones under the ERC Highwind umbrella.
Circular runway poster from AWEC2011 reinhartpaelinkawec.pdf (2.7 MB)
And the full research continued in a thesis by M. Clinckemaillie in 2012.