border-collapse

Scenario 1: table over table

<table style="width:200px; border-collapse:collapse; border:10px solid #F00;">
    <tr><td>Table 1</td></tr>
</table>
<table style="width:200px; border-collapse:collapse; border:10px solid #000;">
    <tr><td>Table 2</td></tr>
</table>

Scenario 2: table over image over table

<table style="width:200px; border-collapse:collapse; border:10px solid #F00;">
    <tr><td>Table 1</td></tr>
</table>
<div><img src="white.gif" height="1" width="100" alt="" /></div>
<table style="width:200px; border-collapse:collapse; border:10px solid #000;">
    <tr><td>Table 2</td></tr>
</table>

Scenario 3: table over table with margins

<table style="width:200px; border-collapse:collapse; border:10px solid #F00; margin:5px;">
    <tr><td>Table 1</td></tr>
</table>
<table style="width:200px; border-collapse:collapse; border:10px solid #000; margin:5px;">
    <tr><td>Table 2</td></tr>
</table>