Trade union collaboration on issues of social justice connects workers to each other and to their wider communities and is therefore considered a strategy of union renewal. Prior to their amalgamation on 1 January 2023, Lärarförbundet (Swedish Teachers' Union) and Lärarnas Riksförbund (National Union of Teachers in Sweden) collaborated intermittently to challenge educational injustices in the highly segregated Swedish school market. Through documentary research, with dialogue as method, this case study explores the policies and politics of this inter-union collaboration. Using 'framing' and 'intermediation' as conceptual lenses, we argue that the two unions collaborated predominantly, but to varying degrees, on bargaining, political, and professional issues. However, these joint actions were conducted in the context of declining union membership, competitive national recruitment agendas, and the rise of grassroots teacher-led 'rebellion' groups campaigning for public education. External solidarity in organising was therefore limited by inter-organisational and intra-professional differentiation and both unions' orientation towards a service model of unionism. Future research might explore the complexity of intra-union collaboration in the newly amalgamated Sveriges Lärare and the extent to which regional branch activism and broader alliances with social movements could promote a more democratic organising agenda.