() cilent-side command fires the server-side code to rebind the grid it is called from, so both grids will perform requests and the first will be cancelled as this is the way AJAX works.
I can offer you several ways to go around this:
1) increase the value of the RequestQueueSize
property of the panel
2) use the RadAjaxManager to perform an AJAX request
for both grids (e.g. you could pass a parameter that will indicate in the code-behind that these two grids need to be rebound) instead of the client-side rebind() method
3) use a regular update panel around both grids and a hidden button inside that will invoke the request to rebind the grids. This button's request can be initiated via the __doPostBack() command by passing its UniqueID as the first parameter. Again rebind() should be called from the server this time.
the Telerik team
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