Yes you can. And you should be able to do it on DL.com without calling in though YMMV (DL.com errors out a lot with eCredits). Once you put passenger information in you can look up eCredit for both people and apply.

Even if you can’t online a competent agent can get that done. The trip I’m on with my wife right now is the exact scenario you describe. I had an eCredit that was substantially less than her eCredit (because I had used part of it for another trip) and they were still able to easily apply the eCredit for both of us to be on the same PNR and by extension for us to get C+ at booking (because I’m PM and she’s only GM so if separate reservations that wouldn’t have happened).

Pre-COVID, I did the same thing a couple of times with eCredits that were of the same value. It should be pretty straightforward for a competent agent.