progressively increases on level (and HP). Can drop your second or third Stego, depends.
The second one is the problem for me. After I finish lvl 40 normal Tyranno, I don't if it's RNG, but it's always a miss. The 2nd Tyranno never appears, or sometimes, needs a lot of time and afk for it to appear. I believe these two Tyrannos can't exist simultaneously in GX world. So why 2nd rarely appears?
Why can't you challenge again? The lvl 100 Tyranno icon will stay in GX duel world, and his life points will carry on from the last duel. Nothing will stop you from clicking it and challenging him again. It does not disappear after you lose against him either.