I created 2 different menus as i need to show menu1 on some pages and menu2 on others.
in order to show both menus on the header I'm using Dropdown Menu plugin. So for example in page 1 I add the Dropdown menu widget selecting menu 1 and then from dynamic widgets I decide which page to exclude. I do the same thing with meu 2 (add dropdown menu widget again to the header by selecting menu 2 this time and then excluding pages from dynamic widgets).
After I save, the second menu2 widget disappears from the header widgets.
I'm quite sure it's not a Dropdown menu plugin problem, because if I don't use dynamic widgets both menus stay in the header.