Assume, I have two interfaces A & B using the same table. Interface A using the seq.nextval to get the ID and insert, and interface B using max(ID) +1. Ofcourse both interacting with the same table. To be consistent, I would like to change interface A to use the above query for a check and balance.
I'd suggest you to abandon "MAX + 1" and never use it in multi-user environment. Switch your code to use of a SEQUENCE and never worry about the problem you are trying to deal with now.
However, if you insist on this solution, it seems that you'll have to use PL/SQL code (a function might be fine), such as
CREATE OR REPLACE FUNCTION Fun_Max RETURN NUMBER IS
SELECT my_seq.NEXTVAL INTO l_seq FROM dual;
SELECT MAX(deptno) INTO l_deptno FROM DEPT;
RETURN (GREATEST(l_seq, l_deptno));
Use it as
SQL> SELECT fun_max FROM dual;