You can use something like this to get the output you have in your question:

string txt = "department_name:womens AND item_type_keyword:base-layer-underwear";
var reg = new Regex(@"(?:department_name|item_type_keyword):([\w-]+)", RegexOptions.IgnoreCase);
var ms = reg.Matches(txt);
ArrayList results = new ArrayList();
foreach (Match match in ms)
{
    results.Add(match.Groups[0].Value);
    results.Add(match.Groups[1].Value);
}

// results is your final array containing all results
foreach (string elem in results)
{
    Console.WriteLine(elem);
}

Prints:

department_name:womens
womens
item_type_keyword:base-layer-underwear
base-layer-underwear

match.Groups[0].Value gives the part that matched the pattern, while match.Groups[1].Value will give the part captured in the pattern.

In your first expression, you have 2 capture groups; hence why you have twice department_name:womens appearing.

Once you get the different elements, you should be able to put them in an array/list for further processing. (Added this part in edit)

The loop then allows you to iterate over each of the matches, which you cannot exactly do with if and .Match() (which is better suited for a single match, while here I'm enabling multiple matches so the order they are matched doesn't matter, or the number of matches).

ideone demo

(?:
  department_name     # Match department_name
|                     # Or
  item_type_keyword   # Match item_type_keyword
)
:
([\w-]+)              # Capture \w and - characters