Only one element may has a particular id. If two or more elements share the same id, then you cannot find the exactly element by id.
A better way is to set the class attribute to every grid, at later you can use the class selector to find them, just like this:
<table class="mytable easyui-datagrid"></table>
If you want to do some actions with the grid that displayed in selected tab panel, get the selected tab panel first and then find that grid:
var p = $('#tt').tabs('getSelected'); // get the selected tab panel
var g = p.find('table.mytable'); // get datagrid object
g.datagrid('reload'); // call datagrid method