The easy way is just to insert another td that's just a pixel wide like this:
<td style="background-color:#ff2020; width:1px"> </td>
or, you could just add the style for border-left to the second td like this:
<table>
<tr>
<td style="width:150px; text-align:center;">Cell 1</td>
<td style="width:150px; text-align:center; border-left:1px solid red;">Cell 2</td>
</tr>
</table>
Either way, the cleaner route would be to use a stylesheet and classes.