how do I make a 'reunion' between 2 lists

[admin]

If I have 2 lists with different nodes, but some of them have the same name, how do I make a third list which contains all the nodes from the first 2 lists but does not have 2 elements with the same name?

EX:

<SWP>
<P0>
...........
</P0>
<P1>
...........
</P1>
</SWP>


<APP>
<P0>
...........
</P0>
<P1>
...........
</P1>
<P3>
...........
</P3>
</APP>

So, in this case if I have list1 with the nodes from SWP and list2 with the nodes from APP, how do I make list 3 which contains only the nodes P0, P1 and P3?