try This :

 <tr>
            <t t-foreach="list" t-as="item">

                    <td><span t-esc="item_index"/></td>
                    <t t-if="item_odd">
                        &lt;/tr &gt;
                        &lt;tr &gt;
                    </t>
            </t>

</tr>

Hello Paul,

As I observed you theory is on opposite way of qweb tag (specially for the ending tag).

But I can understand what kind of output you want.

You need to create list of list.

I am explaining by your example

<t t-foreach="list" t-as="item">

    <tr t-if="item_even">

        <td><span t-field="item"></td>

    </tr t-if="item_odds>

</t>

Here "list" is a list something like [1,2,3,4,5].
and it will print like 
1
2
3
4
5

and you want it like
1   2
3   4
5

If you want something like this, then you need to pass list of list like,
your "list" should be like [[1,2],[3,4],[5]] instead of [1,2,3,4,5].
Now you xml code should be something like.

<t t-foreach="list" t-as="row">

    <tr>
        <t t-foreach="row" t-as="item">

            <td><span t-field="item"></td>
        </t>
    </tr>

</t>

and python code for converting simple "list" this "list of list" is

Option 1 :

your_list = [1,2,3,4,5,6,1,2,3,4,5,6]
new_list = []
for i in range(0, len(your_list), 2):
    temp_list = [your_list[i]]
    if i < len(your_list)-1:
        temp_list.append(your_list[i+1])
    new_list.append(temp_list)
print new_list

Option 2 :

your_list = [1,2,3,4,5,6,1,2,3,4,5,6]
new_list1 = zip(*[iter(your_list)]*2)
print new_list1

I have a similar requirement but with three columns for a pay slip report. 

The solution that I came up with is to construct some part of the Qweb report in python and save the generated html in a text field.

Call the text field in the Qweb report like this:

<div t-field="o.field_name" t-options="{'widget':'html'}"/>

When the Qweb report gets generated, the content of the text field is rendered together with the rest of the report. I use this approach in Odoo13 but I guess will work on previous versions as well.