I have no knowledge about " solartwin " . But check the estif homepage
http://www.estif.org for EU standards to make sure if the product is good and efficient.There is an EU standard number.
The page of solartwin is complicated to get through , I could not find facts for efficiency.
Two panels are very,very small. The minimum you need is about 6m2 for the average household . Since they claim that their system is "flexible" and needs no frost protection the collector could be made of sub standard plastic pipes instead of metal/copper. "Substandard" because plastic is a bad heat conductor, as well it can't be soldered to fast transmitting metall heat collector surfaces. If it is so than stay away from this product , it would take ages to get something out of it .
In Germany the trend of new installations goes to 9m2 for warm water and 11m2 for combined domestic water and heating support.The panels are the cheapest part of the entire installation ( about 8% of the entire price incl.installation and VAT etc.), so they get the most out of it by using as much surface as possible.
Make sure as well that you get a well insulated and large enough storage tank, 300 liters are the absolute minimum.
Some information and sellers and installers are available at
http://top50-solar.de/ ,you will be directed to the English page directly or simply click the "welcome" window. Manufacturers there are quoting prices as well. Before deciding for a system buy a good book about the issue , at least if you are planning to install the system your self.