The LaTeX builder doesn't handle multiple toctrees with captions well

Problem

The HTML builder has good support for multiple separate toctrees with captions, like this:

.. toctree::
   :caption: AAA

   A1
   A2

.. toctree::
   :caption: BBB

   B1
   B2

In Alabaster it's rendered like this:

AAA

  A1
  A2

BBB
  
  B1
  B2

And in the RTD theme A1 and A2 are grouped in the table of contents:

The LaTeX builder, on the other hand, gets confused when presented with the same input:

Procedure to reproduce the problem

mkdir /tmp/sphinx-repro
cd /tmp/sphinx-repro
sphinx-quickstart --quiet --author x --project y
for i in {A,B}{1,2}; do printf "$i\n==" > "$i.rst"; done
cat > index.rst <<< '.. toctree::
   :caption: AAA

   A1
   A2

.. toctree::
   :caption: BBB

   B1
   B2
'
make latexpdf

Expected results

The table of contents should be split into two parts in LaTeX like in HTML

Environment info

• OS: GNU/Linux
• Python version: 3.5.2
• Sphinx version: 1.7.1

Reproduce the problem with the provided sphinx-quickstart and make latexpdf commands, then trace the LaTeX builder's handling of multiple captioned toctrees. Compare its output with the expected HTML-style split and add coverage for the two toctrees so completion can be verified.