Alternative required fields when creating a lead

Hi,

When creating a lead, we need to have the field “Main Phone #” required OR the field “Email” required.
If one of these field is null, then the second has to be filled out. But both can have a value too.
We try to get a solution by using the “IfNull” function or a WF, but it doesn’t work on creation.

Do you have any idea to solve this issue?

Thanks for your help