Miltiple fields in the same column.

Is there a way to concatenate two fields with the same criteria in the same column using SQL statement? There's more then one [Request ID] (Col2) for the same [Invoice Number] (Col1). The desired output is:

Col1 | Col2
[22] | [12 21] <--- these [Request ID]s are from two different records

I tried subquery for Col2 (SELECT ...) but it can only return one record ([Request ID]) at a time.

Thanks.