(As my previous post contains some information other users might find useful I reply to myself)
Hi 3FG, with the kind help of Jon Rhees - the developer of the USB-UIRT device - I could solve the problem.
He has developed the learnhelper
application which allows you to register toggled and non-toggled IR codes separately (which equals the Girder function I mentioned in my previous post). I had to concatenate the two strings with a # character, add the combo-code to the separate IR files Mediaportal's IR Server suite produces and I was done. Jon has provided a how-to
in his USB-UIRT forum.
Thanks again for your very appreciated help.
Should I post my IR codes somewhere?