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This looks like something the soldiers are being trained to do, or were trained to do.  This means procedures were likely worked out that mitigate most bad effects most of the time.  The top picture in particular looks like a training exercise, and both soldiers are wearing obvious hearing protection.

<blockquote>Wouldn't the supporter suffer hearing loss?</blockquote>

This is probably not as bad as it looks:<ol>

<li>It looks like the soldiers are wearing hearing protection.

<li>The loudest noise comes from the end of the barrel, not the side.

</ol>

<blockquote>Wouldn't the barrel burn his ear?</blockquote>

I don't see any barrels touching ears.  In no case is the actual barrel even touching the supporting soldier's garment, and certainly not his skin.  In the top picture, it's not the barrel resting on the soldier's back.  In the other picture, you can clearly see a perforated heat guard around the actual barrel.  I expect those get hot eventually too, but not as hot as the actual barrel.  There is also an insulating layer of uniform between that and the skin.

Again, it is doubtful the army wouldn't have established procedures for this to prevent injury if using a second soldier to steady a long barrel is standard practice.

<blockquote>I'm assuming that no human can sit still enough and the supporter will flinch.</blockquote>

Surely the second soldier will try to be as steady as possible.  Of course there will be some inevitable movement, but that is still better than having one soldier trying to hold a long-barrel rifle while aiming by himself.  If you're worried about flinching as a result of the shot, it doesn't work that way.  The bullet will be well past the end of the barrel before human reaction time to the sound.