Adding salts to total brewing water volume

I use Bru N Water and I'm considering changing my salt additions to ensure I get thorough mixing. Is there a problem with adding both mash and sparge additions to the HLT which has the full volume of brewing water. For this beer the mash and sparge water volumes are almost identical, before I start fly sparging I'll treat the sparge water with phosphoric acid. Also for this beer I don't need to add anything that increases alkalinity.