Reaction between carboxylic acids and amines Would some be able to explain why when I mix a carboxylic acid, like ethanoic acid, and an amine, say ethylamine, I get ethylammonium ethanoate. But when I mix a bi-functional group carboxylic acid (say ethanedioic acid) and a bi-functional group amine (say 1,2-diaminoethane) I get a condensation reaction and an amide bond is formed. 

This also leads to why does ethanoic and ammonia yield ammonium ethanoate and not ethanamide. I understand that the ammonia is acting as a base in this reaction, but why doesn't it favour being a base rather than a nucleophile? 

To me both of the reaction seen the same. Can someone elaborate this for me, or am I missing a key piece of knowledge.
You have a false dichotomy.

Diamines and diacids can form salts with doubly charged anions and cations. Sadly, simply searching SigmaAldrich’s catalogue for the phrase diammonium did not turn up any example; however, I assume this is because these salts are unlikely to be used and thus wouldn’t sell. Chemically, proton transfer is one of the fastest reactions possible and thus simply mixing a diacid and a diamine in a suitable solvent would result in the diammonium diate salt as it would for an ammonium monoate salt.

Amide bond formation is a different reaction that typically requires activation or harsher conditions (e.g. heating). It is a condensation that results in the release of water. All that is required is the presence of an amino group and a carboxylic acid. So aminoethane and ethanoic acid can be heated in solution to produce N-ethylacetamide.

The one difference between the mono and the di systems is that the mono system has no of polymerising or cyclising. So the amide bond will always consist of one acid fragment and one amine fragment while in the case of a diamine and a diacid polymerisation or cyclisation can occur.