how to get filtered multilookup it based another one multilookup, in dynamically in ax2012

In My Form i have two string control (Site and Warehouse).In my string control i had written code for multiselect lookup using Syslookupmultiselectctrl class.Then when i select the Sites in the site control, i will get the Site values from the site control in the warehouse lookup method and i will filter the corresponded warehouse values in Warehouse control. I had placed breakpoint at Warehouse control lookup method. When i choose the site and then when i click the warehouse Control the lookup method method hits at breakpoint. Then after when i changed the site values in Site control , and then when i click the Warehouse control the lookup method for that warehouse control was not hitting at breakpoint. Then i commented the Syslookupmultiselectctrl class line in the warehouse lookup method code, it hits the breakpoint everytime when i click the warehouse control. Why the lookupmethod in warehouse control is not called at second time.So, instead of using Syslookupmultiselectctrl class , is there any other solutions to achieve the multiselect lookup. if second time lookup method is call,my problem is solved. any idea about this