What you're describing is the same as typing the variable as="element()" eg:

<xsl:variable name="root" as="element()+">
  <test1 />
  <test2 />
</xsl:variable>

By adding as="element()+" the variable goes from a root with two child
elements that are siblings (eg to access <test1/> use $root/test1) to
a sequence of elements with no root and that aren't siblings (to
access <test1/> use $root[1])

Cheers,
-- Abel Braaksma