I just recieved my second tello on Thursday. I did the setup for my wifi repeater yesterday and all went well. What I noticed is I think you have to do a complete setup for the repeater each time you switch drones. If there is a way to switch drones without doing this I'm not sure. I looked at ordering a second wifi repeater last night and noticed they have really shot up in price. About 50 dollars now.
IIRC, a few months ago someone lost their Tello and bought a second one. Came here asking about pairing it with the Wi-Fi extender without having to reprogram it. He was advised to rename the new Tello to exactly the same network name as the first one.
That leads me to believe that if you have 2 Tellos named exactly the same, you can use the same extender as long as the two are not turned on at the same time... worth a try?