You can use Schematron embedded rules for that. See below a full
sample:

<?xml version="1.0" encoding="UTF-8"?>
<grammar ns="" xmlns="http://relaxng.org/ns/structure/1.0"
  xmlns:sch="http://www.ascc.net/xml/schematron">
  <start>
    <element name="table">
      <oneOrMore>
        <element name="column">
          <sch:pattern name="Check to have the same number of cells in
each column" id="cells">
            <sch:rule context="column">
              <sch:assert test="count(../column[1]/cell) =
count(cell)">The number of cells in this
                column should be the same as in the firtst column,
expected <sch:value-of
                  select="count(../column[1]/cell)"/> but got
<sch:value-of select="count(cell)"/>.
              </sch:assert>
            </sch:rule>
          </sch:pattern>
          <oneOrMore>
            <element name="cell">
              <empty/>
            </element>
          </oneOrMore>
        </element>
      </oneOrMore>
    </element>
  </start>
</grammar>

On your second document (after you make it wellformed) you will get:

The number of cells in this column should be the same as in the firtst
column, expected 2 but got 1 . (count(../column[1]/cell) = count(cell))

Best Regards,
George
---------------------------------------------------------------------
George Cristian Bina
<oXygen/> XML Editor, Schema Editor and XSLT Editor/Debugger
www.---.com