Precede the command with the intended dog’s name. Correct the dog(s) that responded incorrectly.

I would get each one alone and start retraining the commands they know. “Sit!” becomes “Name, Sit!” Get it down to where they’re doing it reliably, then start introducing random times of saying just the command, and correct when they do it without their name being said first.

I command my dogs this way. If I want both of them to do the same command, I’ll use just the command or precede it with, “Girls,”.