Activating an individual questionnaire only if there are eligible respondents from within a household roster

Good morning
Thank you again for the help you give us when we ask you. I need your recourse to this situation.
Indeed, I have a household questionnaire and an individual question.
The household questionnaire is sent to the head of the household who lists all the usual members of the household. With variables
101. Name of household member / visitor
What is $'firstname’s relationship to the head of household?
103. Is $‘firstname’ male or female?
104. How old was $‘firstname’ at their last birthday?
What is $'firstname’s current marital status?
106. Does $‘firstname’ usually live here?
Did $‘firstname’ stay here last night?
So when a household member is over 15 years old, then he is eligible for the individual questionnaire that opens automatically in ODK (option to edit a form) and the household questionnaire is also backed up in sent a form in ODK.
To complete the individual questionnaire of the eligible, we simply enter an edict form, and we fill out the form. In the end, two separate forms are obtained for this example, including that of the household and individual completed.
So how to create these two forms separately having a link between them.