D2 is kind of a complex steel. Pure theory from a nitwit machinist that knows just enough about metallurgy to be dangerous:
After a little while at welding heat, the A2 won't be A2 anymore, and the D2 won't be D2 anymore. At welding heat, the carbides will be dissolved and the carbon will quickly migrate from the D2 to the A2 until they both average about 1.25% (if my memory is serving me correct). You will then have carbon lean D2 that will require an even higher austenitizing temp to get enough free carbon in solution due to all the alloying (think HSS), and carbon rich A2 that might behave strangely (perhaps not properly air harden - form pearlite?). In that state, most all the carbon in the D2 not tied up in martensite will be attached to vanadium - so there will be a lot of free chrome so I'd guess it would etch less than normal D2 and have good contrast?
I don't know how you would go about spheroidizing this mix - but that, I think, would help redistribute the carbon how it ought to be. The extra alloying in the D2 would attract more carbon, and done right I think you'd be in good shape. The carbon will move freely between the two steels - but the alloying will not, and the alloying will move that carbon in different directions, depending upon the temperature.
Again, this is just speculation. Perhaps Mete will chime in.
