The deduction is taken in the year *made*.

The bunching comes from making the donations in the specific years.... don't make it on 12.31.xx, wait until 1.2.yy.  

Then make the next donation on 12.31.yy.   

So there there you are - *bunched* two years of donations into the year  * yy *.

But you as the preparer don't do this, the client has to manage it.