Required combination of elements, attributes and their values

Question

Here is my xml:

<node>
    <elem pk="1">test1</elem>
    <elem pk="2">test2</elem>
    <elem pk="3">test3</elem>
</node>

In my scenario it is required that my node must contain the elements with pk="1" and pk="2".
The element with pk="3" is optional.

Is there a way to catch this scenario in a XSD schema?

Thanks in advance!