If you are on separate PNRs, you can still put a companion on this list, but their upgrade priority goes to the bottom of this list (and are only eligible for First, not C+). Here's the excerpt from Delta's complimentary upgrades page:
https://www.delta.com/us/en/skymiles...llion-upgrades

• If you and your companion are in different paid ticket reservations that have been linked, upgrades to U.S. 50 domestic Delta One, First Class and Delta Comfort+ work a little differently:
  • For domestic Delta One and First Class, your upgrades will clear based on your individual Member Status and confirmation windows, if available. For example, if Platinum and Silver Medallion Members are traveling together, the Platinum Medallion Member will clear at the Platinum window and the Silver Medallion Member will clear at the Silver window. If your companion is a General Member who does not have Medallion Status, their upgrade will clear no sooner than 24 hours prior to departure.
  • For Delta Comfort+, General SkyMiles Members are not eligible for Complimentary Upgrades. The Medallion Member will clear for their upgrade at their respective window.
  • Linked reservations are reservations that have been booked separately, but have been linked to one another by Delta Reservations. If you would like to request a companion upgrade for a SkyMiles Member that is not already in your reservation, you can call to link your reservations at 1-800-323-2323 in the U.S. or view worldwide phone numbers – be sure to have both confirmation numbers handy. You can also request to link your reservations at the airport.