BEx Mandatory Variable Problem

hi all

I have two mandatory variable in my selection like vendor and customer.

my requirement is like that

" Either vendor or customer is mandatory, not both"

means

1> if we execute the query, and put both the value in selection screen and execute system should give above message and come back to that selection.

2> if we put both the field remain blank system should give same message.

3> if we give any one of them it should execute properly.

thanks

samit