Trapped

[dukealien]

So close to falling in.  Good grabber first line.
Little half-slices peering over the threshold.
Darkness sucking my flashlights faint glow down like a straw.  Apostrophe wouldn't hurt (flashlight's) although a second flashlight might be your reflection
From the air erupts hellacious screaming; it cascades off the cave walls.

From the bottom is my facsimile who stares back up at me, pondering oppositely.  Although "who" here helps personify the Other for later, it might flow better without both "is" and "who" (see below)
I scream down at him and afterwards he screams back up at me.

The quiet is deafening here.
My head is spinning from it.
Suddenly I am falling weightlessly backwards.

From the bottom, I stare up at my opposite.  Comma may not be necessary.
I scream at him but he remains silent.

Very visual - that's good.  The fifth line ("From the bottom is my facsimile who...") is a little problematic as to length and meaning.

In general, you use present participles (-ing) a lot.  This is good for immediacy and placing the whole narrative in the present.  You might consider changing up a few of these as you did in line 5 ("who stares" which could have been "who is staring") for variety.  It also eliminates the need for some form of "is" ("I fall" rather than "I am falling") but there are real advantages because  the way you've done it makes each line very immediate but also a little depersonalized, at a slight remove.

My only other critique is the usual advice to examine all instances of "the" for possible removal.  Save "the" for unique instances:  THE pope but A bishop.

It's a nice concept:  multiple personalities aware of each other but unable to communicate?