What I would do is this:

1) Import the first text file to an empty temporary table.

2) Run an append query to copy the data from the first table to the
permanent table. Include a calculated field in this query that provides the
value of "A" for all records.

3) Delete all records from the temporary table.

4) Import the second text file to the empty temporary table.

5) Run an append query to copy the data from the first table to the
permanent table. Include a calculated field in this query that provides the
value of "B" for all records.

6) Delete all records from the temporary table.