You want to create separate lead records with the same email address.
Example: "John and Mary share the same email address email@example.com. How can they be created as two separate records in Marketo?"
By default, records created in Marketo via the areas listed below are deduplicated against Email Address. This can be configured to include additional deduplication field(s) so that records are deduplicated against Email Address + Additional Field(s)
To set up custom deduplication rules, create a support ticket providing the additional fields that you would like to include as part of the deduplication and which areas you would like to apply it to.