select a.ColumnA, a.ColumnB||':'||b.ColumnB
from myTable a, myTable b
where a.ColumnA = b.ColumnA
and a.ColumnB != b.ColumnB
This will probably return
A1 C1:C2
A1 C2:C1
...
If the Column B values are orderable, you might consider:
select a.ColumnA, a.ColumnB||':'||b.ColumnB
from myTable a...