You can simply cast it to varchar(max)/nvarchar(max), concatenate and then
cast it back.
declare @x xml, @y xml
select @x = '<contact>
<plate>
<plateid>60</plateid>
<actionid>1</actionid>
</plate>
</contact>'
select @y = '<contact>
<plate>
<plateid>59</plateid>
<actionid>2</actionid>
</plate>
</contact>'
select cast(cast(@x as nvarchar(max)) + cast(@y as nvarchar(max)) as xml)