Oracle: Single multicolumn index or two single column indexes

It depends...

It is quite unlikely that an index on just column1 will be beneficial if you already have a composite index on column1, column2. Since column1 is the leading index, queries against the table that have only column1 as a predicate will be able to use the composite index. If you are frequently running queries that need to do a full scan of the index and the presence of column2 substantially increases the size of the index, it is possible that an index on just column1 would be more efficient since the full index scan would need to do less I/O. But that is a pretty unusual situation.

An index on just column2 may be beneficial if some of your queries against the table specify predicates on just column2. If there are relatively few distinct values of column1, it is possible that Oracle could do an index skip scan using the composite index to satisfy queries that only specify column2 as a predicate. But a skip scan is likely to be much less efficient than a range scan so it is reasonably likely that an index on just column2 would benefit those queries. If there are a large number of distinct values for column1, the skip scan would be even less efficient and an index on just column2 would be more beneficial. Of course, if you never query the table using column2 without also specifying a predicate on column1, you wouldn't need an index on just column2.