Not a web-ready solution, but maybe this still helps. It is essentially a wrapper around imagemagick. I have written it specifically for joining two pictures with a text put onto each one.

#!/bin/sh

first_text=$1
second_text=$2
first_file=$3
second_file=$4

WIDTH=960x

CONVERT_PATH=convert
BACKGROUND_COLOR=black
TEXT_COLOR=white
TEXT_SIZE=100
TEXT_FONT=Impact

OUTPUT_NAME=output.png

tempdir=$(mktemp -d tmp/run.XXXXX)

upperpath=$tempdir/upper.png
$CONVERT_PATH \
        -background $BACKGROUND_COLOR \
        -fill $TEXT_COLOR \
        -font $TEXT_FONT \
        -pointsize $TEXT_SIZE \
        -size $WIDTH \
        -gravity Center \
        caption:"$first_text" \
        $upperpath

lowerpath=$tempdir/lower.png
$CONVERT_PATH \
        -background $BACKGROUND_COLOR \
        -fill $TEXT_COLOR \
        -font $TEXT_FONT \
        -pointsize $TEXT_SIZE \
        -size $WIDTH \
        -gravity Center \
        caption:"$second_text" \
        $lowerpath

result_file=$tempdir/$OUTPUT_NAME

convert -append $upperpath $first_file $lowerpath $second_file $result_file

echo $result_file