The use of two stress relieving operations in tandem can be either positive or negative.  It will depend upon the desired end results and exactly how one applies the process. 

As to bringing up curiosity, mine would be why one would bother to use peening after PWHT in the form of stress relieving.  There are cautions in more references than just the D1.1 code about using peening on cover passes, especially since we are not talking about cast iron or other special materials where it is used successfully to control shrinkage differences between the base material and the filler material. 

When looking to stress relief, the end result of a properly run heating process is far superior than peening for end product quality.  You get grain boundaries restored with carbon and other alloying elements equally distributed reducing the heat effected zone to pretty close to zero.

But how the structure was assembled at fit up and welded out will determine the type of stresses being dealt with and what type of distortion may be encountered during an oven stress relieving process. 

Peening will aid more in distortion control during welding while PWHT stress relieving will aid more in a total weldment stress relief after the work is complete.

Both are generally more of an engineering consideration taken away from most fabricators, but if this is a curiosity question with no actual job associated with it and no codes and/or engineers involved then my choice would still be to say PWHT and skip any ideas of peening.  Few welders have the knowledge and experience, let alone tools (peening is not properly accomplished with a slag hammer nor any air operated needle scalers or chipping guns) to properly apply peening anyway.  PWHT takes care of any stresses if controlled correctly for the grade, thickness, and weldment configuration far better than peening. 

Finally, we are talking about A36 in approximately 2" (I am assuming that though you added '+' that it is not that much larger or you would have put 3" + or some such thing).  As such, with proper joint configurations and weldment design followed by adequate preheat and the use of recommended filler metals by welders experienced in distortion control without severely restraining the joint and thus building in stresses you should not have a great deal of stress relief that should be required to have a functional product.  But, since we are talking hypothetical situations on a bet just to see who has their metallurgy figured out the best, I vote for PWHT and skip the peening.  And, don't peen roots and cover passes either way.  Usually causes or at least covers up more problems than it solves.

He Is In Control, Have a Great Day,  Brent

PS, nothing makes it easier to answer anyone's question.  This is a volunteer forum with only personal opinions offered and our time is often wasted answering repeated questions that can be easily researched.  Many of those with posts found when searching for 'stress relief' are far more competent in this subject than I.  And many of the links posted give enough detail that while they may not have specifically answered your question of using two methods or one over the other they would lead a person to make their own best decision for their particular application.  Some pretty good reading if you only covered the first page of results and followed the links.