How can I merge citations using Endnote in Pages?

I am currently writing my thesis with Pages using Endnote literature management program. Now I want to cite 2 or more sources in one citation (e.g., Miller, 2000; Gotthard, 1999), but whenever I try to edit one citation it is immediately replaced by the other and it won't combine these two. I've seen a youtube video where it works but when I try to do it manually, it won't. The URL is https://www.youtube.com/watch?v=_uty8PEOR6Q starting from 1:36. Please help me, I have so many sources that I need to combine and it is just tedious to do it all manually.

I would really love, when Endnote support would reach Apple's attention and that the Program could be further elaborated. This just as a sticky note. Thanks all in advance