Two users view a record at the same time and use the value for the 'next' value.

Please let me know if this question belongs under another forum.

We are issuing a set of sequential numbers. The next number issued is determined by the last number issued. So the user looks at the 'ending' number field in Table A, then inputs that number +1 as the starting number in the appropriate field in Table B, then enters the 'ending' number in its field in Table B . When the record is committed the 'ending' number in Table B is written into the 'ending' number field in Table A.

The question is: how do I keep a second user from seeing the same initial number in Table A (before the first user has finished and committed her record) and assigning a duplicate set of numbers?

Thanks