This study builds on two previous studies by the principal investigator, which found that participation in an established and growing "dual-focus" mutual aid model (Double Trouble in Recovery; DTR) by persons with co-occurring substance use and psychiatric disorders was associated with drug/alcohol abstinence and improved psychiatric outcomes. However, these findings are limited as they are based on studies using an observational design with existing DTR groups and a pre-post design with a historical control. This new application seeks to confirm and significantly extend this research by conducting a randomized clinical trial (RCT) of DTR at multiple treatment sites.