Umm, bull****! For example, mashing water for a very pale grist has to have an addition of some sort of acid, while the typically near zero alkalinity of RO requires no acid. Conversely, if mashing a dark grist, the mashing water alkalinity is likely to need to be above zero and the sparging water still doesn't need an alkalinity adjustment.
Horsefea*****! In neither case would it be necessary to treat the mash and sparge water separately. This, of course requires a properly formulated grist.
Mashing water and sparging water should be treated separately under most brewing scenarios.
Except, for example, commercial breweries that don't do it. See, for example, Kunze (who just died last week, BTW) 3.2.1.8. I don't believe I have ever found it necessary to treat sparge water separately.
That will produce a better result than assuming all water needs to be treated to a single specification.
I don't assume that. I assume that one should do what one needs to do in order to obtain the best beer and thus control (or at least check) mash, wort and fermenter pH. That doesn't include separate treatment of sparge water.
Having stated things somewhat strongly in order to draw sharp contrast it would be well for me to explain in a little more detail the philosophy behind this position.
1)It is a PITA to have to separate out and treat sparge water separately so I don't do it. When my HLT is full the water is 36" deep. I dissolve 28 grams of CaCl2 in 72 mL of water and add 36 cc to the HLT. When it is time to top off the HLT I simply add 1 cc of the solution for each inch of water I add.
2)Just because I write extensively about how to precisely calculate the amount of bottle acid one needs to add in order to hit proper mash pH doesn't mean that I use bottled acid in my brewing. I happen to think the Germans make some pretty good beer and they are forbidden to do so. Instead they get the acid from natural sources such as colored malts and sauermalz and that's where I get mine. IOW I'm a bit of a thrall to German tradition.
Now, dear reader, there is no reason why you need to adhere to the German traditions or use my trick to prepare your brewing water. In fact I'd guess that most of you are in thrall to the producers of IPAs and use phosphoric acid to set mash pH. Were I to do that I'd add enough acid to the concentrated salt mix to zero the alkalinity of the water so that the 1 cc/inch maneuver would still work. I'd then add the additional acid to move the mash to the proper pH to the mash, as commercial brewers do. But I'd put it into the water in the mash tun before I added the grains to that water.
Note that neither RO water nor tap water pre treated with enough acid to bring it to mash pH needs any further treatment for sparge.