It depends on what you want to want to display, but couldn't you achieve what you want by adding one row to the multirow per column/value pair and group by the original rownumber?

E.g.

set original_rownum 1
multirow create my_multirow original_rownum column value
db_foreach ... {
  multirow append my_multirow $original_rownum col1 $col1
  multirow append my_multirow $original_rownum col2 $col2
  incr original_rownum
}

<multiple ...>
<tr>
<group column=original_rownum>
<td>@my_multirow.value@</td>
</group>
</tr>
</multiple>