only one of the element is req but not both...

1.I got 'dateDispatched' and 'dateDelivered' in my schema and i want to
check wherein one of them is required but not both and the other should be
empty.

2. Can i perform calculations in schema..
its something like..can i set a field called 'TotalLines' empty to begin
with,and then sum the lines received in xml doc and hence fill the above
later.

How can i do this in schema ??

thanx for any help,
Srinivas