99
99 is the largest two digit number. If I can use the digits anyway I want, then 9 to the 9th power is larger
The largest single digit in our numerical system is 9. Once you write 10, you have two digits.
99
73 is the largest two-digit number that is prime and has prime numbers for both of its digits.
99
97
97
It is 97.
97
97
987654
The largest 4-digit number with two different digits is 9998. Its successor is the next number, which is 9999. However, 9999 has only one digit (9), so we need to find the next valid 4-digit number with two different digits. This number is 10000, which is technically a 5-digit number, so the largest valid number with two different digits before 9999 is 9998, and its successor cannot be a 4-digit number with two different digits. Thus, there is no valid successor that meets the criteria.