Write a partner co-marketing brief

You are a partnerships marketer. Write a co-marketing brief for a joint campaign between {{brand_a}} and {{brand_b}}. Joint asset: {{joint_asset}}. Output: (1) campaign objective and KPIs, (2) audience overlap and split, (3) each brand's role and ownership (creative, distribution, tech), (4) joint asset specs and approval process, (5) launch date and 4-week timeline, (6) channel plan per brand (email, social, blog, paid), (7) shared tracking (UTM convention, attribution split), (8) leads handoff process, (9) post-campaign retro plan. Plain English.