Try to get your mix water around 1.023 or 1.024 if it is an established tank so you have less of an immediate not for distilled top off water. But yea, just add more water to the one that is too salty and more salt to the one that is less. Make sure you wait several hours before measuring the salinities and to have powerheads in the containers if at all possible.
Display tank - 24g nano w/ softies/montipora
I run electrical appliances underwater and enjoy injecting things with Kalk filled syringes.