HtmlNodeCollection inherits IList<HtmlNode> which inherits IEnumerable<HtmlNode> on which you can call the Enumerable.Concat() extension method to create a new enumerable containing both sources. See How to concatenate two IEnumerable<T> into a new IEnumerable<T>?.
You can also just select both sets of nodes by using an "or" in your Xpath expression:
//div[contains(@class,'butik-large-image') or contains(@class,'butik small left')]
Please note this contains() expects given classes in the given order. If you don't want that, use parentheses and and:
//div[contains(@class,'butik-large-image') 
or 
( 
    contains(@class,'left') and
    contains(@class,'small') and
    contains(@class,'butik')
)]
See How can I select an element with multiple classes with Xpath? for a proper implementation of that and, as above code will match false positives because it doesn't check for the classes as separate words.