I think the only way probably is to work on your end of things. I know it's easier said than done, but rather than limit their interactions, if you can find ways to make things better on YOUR end, that is, things that YOU do while they're busy, or things that you do with him/her to remind yourself you're important to them, etc.
Jealousy is one of these feelings that just gets worse the more you focus on it. You can't wait for it to go away. You need to kill it, and sometimes the best way is to automate a reaction. For instance, make a list of things you really like doing. If you start a jealousy episode, pick one in the list, do it, and try and force yourself to focus all of your attention on it.
I'm suggesting this because I know it has worked for some people, but it requires a lot of effort. The point is to take your minds off of it to avoid it getting worse and worse. You'll still have to deal with the source of the jealousy. And the source of the jealousy is NOT the fact that they spend time together. That's the trigger. If you remove that, you'll still have the problem, just not the symptoms. However I can't tell you what the source really is, from what you say it sounds like a fear of abandonment/exclusion? Do you have that fear in other contexts?