Are there any issues with two different people each flying a Tello drone at the same time right next to each other? Also, is there any issue if both are using the Xiaomi wifi extender? I will be doing this with a friend soon.
multiple wifi can coexist very well so no worries.
Just make sure you do what you can to reduce other type of RF interference: separate the wifi repeaters a few meters, and stay away from anything bluetooth, also switch off BT on your phones.