Why can’t I resize partitions in Disk Utility @ El Capitan

I’ve created second partition in Disk Utility under El Capitan, and installed Yosemite to it. Now I want to decrease size of El Capitan partition and increase size of Yosemite partition, but for some reason I cannot do that, the GUI of new Disk Utility won’t let me to increase Yosemite partition in favor of decreasing El Capitan partition. When I decrease El Capitan partition, it creates another “Untitled” partition that I don’t need at all. Disk Utility under Yosemite doesn’t work with partitions at all, all the data is gray-colored and I can change nothing there. What should I do?