I agree that the two cannot be separated.
The Bible tells us that God works in us to conform us to the image of His Son:
For those whom he foreknew he also predestined to be conformed to the image of his Son, in order that he might be the firstborn among many brothers. And those whom he predestined he also called, and those whom he called he also justified, and those whom he justified he also glorified. Romans 8:29-30
And I am sure of this, that he who began a good work in you will bring it to completion at the day of Jesus Christ. Phil 1:6
However, I also do not think that we are to be passive in this process. The NT often tells us to flee temptation, to use our spiritual gifts, not to quench the Spirit, etc. and of course pray. God uses these things to make us grow in our faith. Does that make sanctification synergistic? I guess it may depend on how you look at it. Personally, I think, whatever part I may have in it, anything I do to increase in godliness is first and foremost a result of His work in me - drawing me to walk more closely with him, encouraging me to put into action the gifts He has given me, calling me to know Him more and more.