jQuery Grid Plugin - jqGrid

HI:

For a period of time I need to have 1 of 2 fields enabled.  I have changed the editoptions to enable the required attribute as follows

editrules:{required:true}

How do I change one on the fly to be required and the other to not required - and then swap them back if necessary.

I need to change this on the fly depending on the type of tax collect(one or the other, but one of the fields must have a value)

Nathan