Conditional Data Enrichment for Leads in Clay: Email and Phone Waterfalls

I'm enriching person-level data in Clay. If a row has an email but no phone, I want to enrich the phone. If it has a phone but no email, I want to enrich the email. When a new lead enters my CRM, I want to enrich it accordingly. How can I conditionally run an email waterfall when the email is blank and a phone waterfall when the phone is blank?