Q. One of our dogs becomes anxious when my husband takes our other dog outside to use the bathroom.
A. Sounds like some separation anxiety is starting. You'll have to figure out for sure if she reacting to your husband's or the other dog's absence. You should try and keep her distracted while the other dog is outside, at first keep the other dog outside for just a few seconds. Reward the other dog (the stressed one) for being calm, bring the other dog back inside so she learns that she always comes right back in. Keep repeating, building up the amount of time she stays out each time.
If you aren't able to work through this yourself you will need professional help and guidance to work through this issue and give you the tools to be effective in helping her through this. This is imperative that be it addressed now. Over time, the behavior re-enforces itself and it gets worse and harder to treat.