RequiredIf attribute to checks if one out of two properties has a value

Hi,
I would like to make a property (called propertyOne) required if one out of two other properties has a value. Please can someone point me in the differenction of a custom written attribute which checks this? I have a required if, but it only lets me check it against one property, when what I need to do is check of one out of two properties has a value and if it does, make propertyOne required on the form.
Any help on this would be appreciated.
Thank you,
Hana
...