In your above example page please wrap the 'tt2' in separate div and apply your css class 'tt2_class' to that div and in 'tt2' data-options set fit property to true.
<style>
html,body,form {
height: 95%;
}
.tt2_class {
height: calc(100% - 100px);
}
</style>
<body>
<div id="tt1" class="easyui-tabs" border="true" style="height:100px;">
<div title="Tab1">
Hello 1
</div>
</div>
<div class="tt2_class">
<div id="tt2" class="easyui-tabs" border="true" data-options="fit:'true'"/>
<div title="Tab2">
Hello 2
</div>
</div>
</div>
</body>
Or if you want to do it with javascript try this.
var tt1_height = $('#tt1').tabs('options').height;
$('#tt2').tabs({height:$(window).height()-tt1_height});
