Simple depends on your skill set....
first, read the data into SAS datasets.
second, write a SQL SELECT statement to do the join.
third, address records in a with two or more matches in b.
Three simple steps. But if you have never used SQL it is not so simple. Something like
SELECT a., b., complev(on a and b)
FROM a, b
WHERE MIN(complev(on a and b)) > 0;
I've not tried this code, but that is where I would start.
Note that, as MIN is a summary function and used in the WHERE clause, this is a Cartesian product "under the hood" so it does not scale well. It's OK for 100x200, but would take forever for 100,000x200,000.