From Act and Being: Towards a Theology of the Divine Attributes:

And yet the gospel account appears to require at least two wills somewhere, as crucially in Gethsemane. When Jesus says, `not my will, but yours be done’, the gospel appears to imply that it is at least conceivable that the Son will will other than his Father. To avoid the problem of there being two wills in God, two were attributed to Christ. There must be in Christ himself, it was argued, two wills, a divine will and a human will, and what we see in Gethsemane is the human nature’s will accepting that of the divine nature.