Automate swapping position of two external displays with displayplacer. You will need to customise your builtin ID and displayplacer commands.

swap-displays.sh

#!/bin/bash
BASEDIR=$(dirname "$0");
CONFIG_FILE=$BASEDIR/SWAP_DISPLAY_CONFIG;
DISPLAY_CONFIG=`cat $CONFIG_FILE`;

if [ -z "$DISPLAY_CONFIG" ] 
then DISPLAY_CONFIG=0;
fi

if [ "$DISPLAY_CONFIG" == "0" ]
    then DISPLAY_CONFIG="1";
    else DISPLAY_CONFIG="0";
fi

echo config:$DISPLAY_CONFIG
# record the current config to file
touch $CONFIG_FILE;
echo $DISPLAY_CONFIG > $CONFIG_FILE;

# Find screen ids
dplist=`displayplacer list | tail -n 1`
ids=`displayplacer list | tail -n 1 | egrep -o 'id:[A-F0-9-]+' | egrep -o '[0-9A-F-]{36}'`

# Sort the ids so they are in consistent order
# https://stackoverflow.com/questions/7442417/how-to-sort-an-array-in-bash
IFS=$'\n' ids=($(sort <<<"${ids[*]}")); unset IFS

for id in ${ids[@]}; do
    if [ "$id" != "$builtin" ]
    then 
        if [ -z "$screenone" ]
        then 
            screenone=$id
        else
            screentwo=$id
        fi
    fi
done


builtin=34CF6778-5B8D-0DE8-96DF-EB47172E8121
echo builtin:$builtin
echo screenone:$screenone
echo screentwo:$screentwo

case $DISPLAY_CONFIG in
0)
displayplacer \
"id:$screentwo res:2560x1440 hz:60 color_depth:8 scaling:off origin:(0,0) degree:0" \
"id:$screenone res:1440x2560 hz:60 color_depth:8 scaling:off origin:(-1440,-540) degree:90" \
"id:$builtin res:1792x1120 hz:59 color_depth:8 scaling:on origin:(402,1440) degree:0"
;;
1)
displayplacer \
"id:$screenone res:2560x1440 hz:60 color_depth:8 scaling:off origin:(0,0) degree:0" \
"id:$screentwo res:1440x2560 hz:60 color_depth:8 scaling:off origin:(-1440,-540) degree:90" \
"id:$builtin res:1792x1120 hz:59 color_depth:8 scaling:on origin:(402,1440) degree:0"
;;
esac