Using qres and a cmd this script will switch from any specified resolution to the other and back just with running the BAT file.

@echo off

rem Get the current resolution
set "zzTmp=desktopmonitor get ScreenWidth^,ScreenHeight"
for /f "tokens=4,5 delims=. " %%a in ('ver') do set "version=%%a%%b"
if version GTR 62 set "zzTmp=path Win32_VideoController get CurrentHorizontalResolution^,CurrentVerticalResolution"
for /f %%s in ('wmic %zzTmp% /value ^|find "="') do set "zZ%%s" 1>nul 2>nul
if defined zZCurrentHorizontalResolution set "zZScreenWidth=%zZCurrentHorizontalResolution%"
if defined zZCurrentVerticalResolution set "zZScreenHeight=%zZCurrentVerticalResolution%"
if not defined zZScreenWidth set "zZScreenWidth=1024" & set "zZScreenHeight=768"
set /a zzSsW=%zZScreenWidth%
set /a zzSsH=%zZScreenHeight%
set "zzTmp="
set "zZScreenWidth="
set "zZScreenHeight="
set "zZCurrentHorizontalResolution="
set "zZCurrentVerticalResolution="
echo. Resolution : [%zzSsW%]x[%zzSsH%]

rem Check the resolution and change it accordingly
if "%zzSsW%"=="2560" (
  qres /x:1920 /y:1080
) else if "%zzSsW%"=="1920" (
  qres /x:2560 /y:1440

Download QRES from here:

look for qres.exe and place it in the same folder as the BAT file.
Run the bat file with the previous code and it will switch from 2560 to 1920 and back.