RE: Import Leads & Contacts At Once?
Hi Burly,
You could take following method as reference:
1. I assume that you had a lead view and it had contained necessary fields as columns.
Here is my view for import.

2. Create a custom lookup field(lead) for Contact entity, and add it contact form so that we can check after import done.

3. Create a temporary workflow at organization level:
entity: lead,
Starts when record is created(leads not exists before import)
create contacts from leads fields.

set FIrstname and Lastname in Event information section.(because fullname field is not editable)

set the custom field to Lead(Lead)

4. After import work has been done, then check whether all related contacts have been created successfully.

If above steps go well, then delete or deactive the workflow.
Result 1:
We can see associated contacts in lead form.

Result 2:
We can see originated lead in custom field of contact form.

In addition, you can also bulk qualify leads to contacts, it's an automatic process, but it'll also generate related opportunity records of leads, you may delete them manually.

Regards,
Clofly