I’ll use Sandi for the jealous dog, and Captain as the other – and of course you can use this process in either direction you need to in order to fit the situation.
1. When both dogs are calm and relaxed, and have some distance from each other (you may need to assist with the distance part), put Sandi into a sit/down stay and give her a treat, then go pet Captain for a few seconds. After giving attention to Captain, go back to Sandi, give her a treat and brief praise, then release her from her stay. Repeat that process a dozen times or so. Gradually increase the amount of time that you spend with Captain, and always remember to visit AND treat Sandi after the interaction.
2. Slowly start decreasing the distance between the two of them as you continue this practice. When you decrease the distance, go back to very short visits with Captain and gradually build up the time as you practice that distance.
What you're looking for is for Sandi to be calm and relaxed about you giving attention to Captain. She'll learn that each time he gets attention, hers is soon to follow, so there's nothing to be jealous about.
Don't rush through this. Let her go at her own pace, and don't move on to the next phases of the exercises until you can see that she's relaxed at the time that you're giving your attention to Captain.
Also, during your "non-training sessions," pay more attention to Sandi when Captain’s around. So, if you're petting Captain, and Sandi comes around, pet them both, switching back and forth between them. Give Sandi lots of verbal praise during the time that you're petting Captain. If Captain decides to leave, immediately stop petting Sandi and quietly walk away.