$('#myTable tr:last').after('<tr>...</tr><tr>...</tr>');
//OR
$('#myTable > tbody:last-child').append('<tr>...</tr><tr>...</tr>');
$('#myTable tr:last').after('<tr>...</tr>');
<table id="myTable">
<tbody>
<tr>...</tr>
<tr>...</tr>
</tbody>
</table>