You could try getting the same effect with floated divs. Create a container div the width of the "table", then have the Ditto template create divs and in your CSS have them be set to a width of about half of your container’s width, a specified height, and floated left. That way it will float two per row and drop the next set of two down to the next row.
If you really want tables, start it like this:
<table><tr>
[[Ditto? ... &tpl=`dittoTpl` &tplAlt=`dittoTplAlt` ... ]]
</table>
Then have your tpl chunk
<td>[+whatever-placeholder+]</td>
then your tplAlt chunk
<td>[+same-placeholder+]</td></tr><tr>
If you don’t have an even number of items it will not have the final </tr> but it should work anyway; it just won’t validate in that case. You could also put the last </tr> after the snippet call, along with the closing </table> tags, and in that case you’d have duplicate </tr> tags if you do have an even number. Again, it should still work, it just won’t validate. You might be able to use phx somehow to work this out.