What is the relationship between synthesis and decomposition reactions?

they are the same process.

they are reverse processes.

they create the same products, although the reactants may be different.

they both start with two elements or compounds.

They are reverse processes. Synthesis is combining one or more element/compound to form a new one. e.g photosynthesis 
Decomposition is the opposite of that. It's when an element/ compound decomposes into it's component elements or simpler compounds
e.g 2H2O (l) --> 2H2 (g) + O2 (g)