Using both pre-modifier and post-modifier on the same variable at the same time is completely valid and is allowed.
For example, the statement
b++=++a++;
is valid, and is equivalent to the sequence: “b=a+2, a=a+1”.
Even pre-increment (not post-increment, though) of numeric constants is allowed when they are used in expressions:
++b++=++1+(++a--)+(++1);