The $20/month plan includes 1TB of cloud storage, which comes with Lightroom Classic, Lightroom CC (cloud), and Photoshop.
If you both use the cloud based Lightroom, your images will get mixed together since it's the same account. If one or both use Classic, the images will stay separate.
Short answer - as long as you both don't use Lightroom CC (cloud version), your images will stay separated.