Validation - Custom callback not behaving as expected?

[eluser]jppi_Stu[/eluser]
Came across this thread almost two years later, while learning about form validation with CI and needing a similar solution (at least one of two fields must be filled, but neither by itself is required). Using code similar to the OP's code, but with the current form_validation library, seems to work. It was necessary, however, to call it only on one of the two fields, otherwise the message would appear once for each field (which makes sense, of course).