[eluser]tonanbarbarian[/eluser]
you are better off using 2 callbacks, one for each field.
from there you have a couple of options
1. the first call back can store its data as a property of the controller and the second callback can grab the first data and its own and process
2. each callback only validates it own data. you should be able to right a model method that can be used by either to valid to reuse code tho
you might even have each of the callbacks call a third private method that takes the value and a flag indicating which type to check to even further reuse code
if none of that is really what you want then just use the 1 callback and put it only on the 1 field, not both
then in the callback use the input->post to retrieve the second value and test it as well
there may be even more options but i cannot think of them at this time