Can you help us figure out why our shih tzu is pooping in the house?

I'd suggest that you need to establish a scheduled routine with structured potty times for your Maltese-ShihTzu to eliminate. If someone is usually home with the dog, using the leash as a tether to the human (wherever human goes, the dog goes) may provide the opportunity for someone to address the "got to go!" signals and take the dog to an appropriate place to eliminate- whether that's inside or outside the home. Often this process is easier if you use a crate whenever not 100% supervised. However, some people prefer to use a playpen indoors or gate off a section of their home with the most easily cleaned, durable floors.

Please read the tips provided at the link below to achieve successful outcome. https://www.humanesociety.org/resources…

Thank you for acknowledging that it's your responsibility as the dog's parents. You may want to ask your husband if he'd give away your human baby if the potty training process didn't go as expected.