Solution
This is what I ended up with using {cycle}
to iterate trough every 2 items, and then closing and opening the tag. As well as making sure the last item does not end with an opening tag:
<ul>
<f:for each="{field.item}" as="item" iteration="iterator">
<f:if condition="{iterator.cycle} % 2">
<f:then>
<li>item {iterator.cycle}</li>
</f:then>
<f:else>
<li>item {iterator.cycle}</li>
<f:if condition="{iterator.isLast} != 1">
<f:then>
</ul><ul>
</f:then>
</f:if>
</f:else>
</f:if>
</f:for>
</ul>
Note
I understand that with the power of VHS v:iterator.chunk
would probably solve this with a little less code, but I could not get it to work how I wanted to. If anyone would be willing to provide a working example to compare the 2 options I would be grateful.
With VHS
Update: 21.12.2017
So revisited the chunk option, and the same code is achieved with the following snippet using VHS iterator.chunk:
<f:for each="{field.item -> v:iterator.chunk(count: 2)}" as="col" iteration="cycle">
<ul>
<f:for each="{col}" as="item">
<li>
test
</li>
</f:for>
</ul>
</f:for>
Perhaps this helps someone out there!