Relatives play an important role in mental health service users’ care. Interventions directed either at service users or their relatives may influence the other person as well. The project Activa’t per la salut mental (Get active for mental health) consisted of a series of four interventions addressed at people diagnosed with mental disorders and their relatives to help them in their recovery process, increasing their agency and quality of life. The objective of the present study is to evaluate the interaction of the participation of service users on their relatives’ outcomes and vice versa. The impact of the project was evaluated within an RCT. The treatment group had access to all the circuit interventions while the control group received treatment as usual and could only access one of the interventions. All participants were evaluated at baseline, six months, and twelve months after the end of the first intervention. Service users were evaluated with the Stages of recovery questionnaire, and relatives with the Family Burden Interview Schedule II and the Duke-UNC-11 questionnaires. The interaction between service users and their relatives was analysed by means of correlational analyses within the intervention group. Service users baseline characteristics influenced in the level of participation of relatives and vice versa. The results also indicated an interaction between service users’ recovery score changes on the change of care burden of relatives. Service users’ participation levels interacted with the decrease of relatives’ burden. These results can be extremely helpful in fostering interactive benefits in future projects addressing the wellbeing of mental health service users and their relatives. Future studies could use specific designs to explore the directionality of the causality of these effects.