It would be much easier to help you if you posted some code demonstrating a minimal test case.
I suspect you've put the instances of your toolbar items on the prototype so that they are shared. e.g. If you do something like this it won't work:
It is important to realize that the tbar array is added to the prototype of the class so the same instance of the combobox will be used by all instances of A.
If this is your problem then you will either need to create the tbar in initComponent() or specify it using xtypes instead. The xtype approach is neater but not as flexible.