Yes, this seems to be the meaning of a "double tax agreement" for most of Europe - I have worked with Germans, French, Belgians, Dutch Spanish etc, all of whom only pay tax at the rate of the country of the employer, even it is way less than that of their home base, on the proviso that they are less then 183 days in their home country.
Unfortunately, the UK taxman has a very different interpretation of a 'dual tax agreement'; If you pay tax at a lower rate than the UK, in the country where your employer is, then you have to pay the diffreence in the UK up to the level of UK tax. Only if you average less than 91 days in the UK per year, AND have a proper non-UK residence will you escape paying tax at the UK rate.
So my question is, are there any other countries which are moving down the lines of the UK - I am curious about the problems the French at Netjets are having, as under practices applied previously, I wouldn't have thought they would be hit for tax over and above that levied in Portugal.