I have heard ever since i little boy that foreign powers wanted to unite both countries for a multitude of reasons and every time there is a feud between the two (like the recent Constitutional Treaty's nationality resolution) and any country makes a say on the matter, Dominican media makes a patriotic explosion regarding sovereignty and "foreign powers attempting to merge the two countries when they have nothing in common", which is also a widely held notion in the general population.
Is there any truth to this or is just propaganda?
Completely different countries in language and culture. Considering the massacre of around 17,000 resident Haitians by the Dominican Republic's dictator Trujillo in 1937, and the lunacy of the Papa Doc Duvalier regime of the 1950's-70's, it is surprising they have not been at war, let alone discussing unification.