Impossible to prepopulate multiple values in select

I have read through the code of the module and it appears to me that there is no way to prepopulate multiple values in a select list. The module just assign whatever is passed in $_GEt to #value of the corresponding element of the form. If one needs to populate multiple values in a select list the #value should be equal to something like that array('Value 1', 'Value 2'), but there is no way to pass this via $_GET.