Have you tried simply using a pair of 8-ohm speakers
with the amp? Chances are they'd work (less power,
but shouldn't harm the amp).

I have, but then the audio output is too quiet for my requirements.

I'm not being sarcastic -- but how do you know that paralleling the
outputs will produce a significant increase in power? Ignoring the
very pertinent question of how to do this /correctly/, the most you
could get is a 3dB increase, which is plainly audible, but hardly
a large increase.

A small increase is better than nothing.

No, it isn't. Not when it requires a lot of work, and will be only slightly
audible.

"Forget about the whole thing." -- The Lady from Philadelphia