how to add multiple key value pairs to dictionary Swift

This is how dictionary works:

In computer science, an associative array, map, symbol table, or dictionary is an abstract data type composed of a collection of (key, value) pairs, such that each possible key appears at most once in the collection.

So

var cart = [String:String]() 
cart["key"] = "one"
cart["key"] = "two"
print(cart)

will print only "key" - "two" part. It seems that you may need an array of tuples instead:

var cart = [(String, String)]() 
cart.append(("key", "one"))
cart.append(("key", "two"))
print(cart)

will print both pairs.