Blynk only allows an Auth token to be used on one device at a time. If you have two devices sharing the same Auth token then when the second device connects to Blynk, the first will be kicked-off.
You’re doing something wrong with your Arduino OTA then. Blynk and Arduino OTA work well together, provided you write your code correctly.
Automations, the HTTP(S) API and Node-Red are all possible solutions…
Pete.