You could have Opus display one of its dialogs however if it were me since this is a batch file, I'd use a batch file script popup like the free one Frank Westlake offers here:
maem.umr.edu/batch/f_w_util/popup.zip
If you grab that above popup program and unzip it, just change this line in your script from:
if not "%~3"=="" goto:end to something like this (you would have to change the path to popup.exe to where it would be on your computer):
if not "%~3"=="" (
"D:\Other Tools\PopUp.exe" /ok /m:"Only 2 files can be selected!"
goto:EOF
)
By the way in case you weren't aware of it, once you have your batch file scripts working exactly as you want, you can prevent Opus from showing the DOS box flash by putting @runmode hide in your button command like this:
@runmode hide
"D:\Mine\cmd\Test.cmd" {F}
As far as VBS goes, while I've written a number of VB scripts in IMatch, I've barely touched them in Opus yet (old habits of using batch files or 4NT scripts die hard for me) so I'll have to pass (at least for now) on the vbs version of this.