@chad3006,
I'm not aware of any instances where languages have peaceably merged. As far as I know, one language normally colonizes another in the cases where two language groups occupy the same area. They often seem to have merged because the language adopted tends to take on features, and vocabulary of the languages not adopted as primary. Take English for example, it no longer structurally resembles its Germanic roots because of the Norman occupation which mandated French as the primary language, so once English took hold again it no longer sounded like Dutch and Frissian. This is called superstratum influence. It is when a dominant language group affects a non-dominant language that remains a viable language. In fact English grammar was sort of leveled out almost to an isolated or super charged version of a creole.
Also take Romance languages. Vulgate Latin was spoken in most of the areas where Romance languages are now spoken. It was imposed on the occupants who were Celtic in France and Spain and Slavic in Romania. The local languages affected the Vulgate latin in ways that changed the grammar slightly and the vocabulary significantly. These are called substratum influences.
Granted after so much history and conquering this and economically conquering that etc.. most languages have considerable super and substratum influences from a variety of languages and families.
This leads to pidgeons and creoles. A pidgeon happens normally when two groups meet for trade or a colonizing force does not mandate that the native population learn the colonizing language, but still does not bother to learn the native language. A pidgin is a super simplified language with very little to no real grammar and a mix of both language's vocabulary, normally the colonizing or dominant business force's vocabulary provide the major portion of non-core functions and the native language provide for the core functions. For example Brava Island and Guinnean creoles have native pronouns but most vocab is Portuguese.
there is debate about when a pidgeon turns to a creole but the general consensus is that it happens when a generation of speakers learn the pidgeon as their primary/native language. At this point grammar is naturally added, simple tenses and aspects are added, as well as deictic and other pragmatic systems. It is still a super simplified grammar but it is complex enough to function as a real language.
Don't know if this helped, but its the answer I have.