No 'align' property is available in treegrid plugin. To solve this issue, you can declare two <div> elements and place the treegrid components to each of <div> elements.
<div style="width:49%;float:left">
<table class="easyui-treegrid" style="width:100%;height:250px">
...
</table>
</div>
<div style="width:49%;float:right">
<table class="easyui-treegrid" style="width:100%;height:250px">
...
</table>
</div>