No, you cannot use a Delta Companion Certificate for a multi-city flight in 2026. The terms and conditions for certificates issued via Delta SkyMiles Platinum or Reserve American Express cards strictly mandate that the itinerary must be a standard round-trip flight. According to Delta’s 2026 rules, stopovers, "open jaw" tickets (flying into one city and out of another), and circle trips are not permitted. Both the primary traveler and the companion must be booked on the exact same flights, dates, and class of service at the same time. The certificate covers the base fare for the second passenger, but you are still responsible for government-imposed taxes and fees, which typically range from $22 to $80 for domestic flights and up to $250 for eligible international routes to Mexico or the Caribbean.