Parashas Naso · Passuk 5:16
The priest shall bring her forward and have her stand before GOD.
Chabad.org — English translation + Rashi
Source: Sotah 7a.
English: “And the kohen shall bring her near” — our Rabbis taught: Two sotahs are not given to drink at once, as it is said “and the kohen shall bring her near” — her alone.
In context: The singular ensures each woman faces the ordeal without the emotional support (or distraction) of another suspect.
Custom
Chazal prohibit administering the bitter water to two women simultaneously.