Squatting, or the use of property without the legal permission, and tenants’ activism are under-researched areas, in particular in the post-socialist context. Poland has been pointed out as extraordinary on the map of squatting in post-socialist Europe and a considerable number of tenants’ organizations are active in the country. What is most interesting is that squatters’ and tenants’ activists are forming alliances, despite their obvious differences in their organizational models, social composition, along with the specific motives and goals of their activism. The objective of this article is to examine the relations between the tenants’ and squatting movements in Poland by studying two cities where both movements are established and closely cooperating. In particular we are interested in the transformative power of such cooperation and we assume that cooperation between social movements results in negotiations and transformations of the social movement actors involved. The empirical foundations for this article are 50 interviews, whereof 30 interviews conducted in Warsaw with squatters and tenants’ movement activists and 20 interviews conducted with activists in Poznań. Warsaw and Poznań are, moreover, two Polish cities where the squatting movement is most vibrant and where squatters and tenants have achieved some considerable successes in their activities. The article argues against previous studies emphasizing access to abundant resources and identity alignment as crucial for the mobilization of collective and collaborative action. Instead, it argues that the lack of resources might also be driving social movements towards cooperation, as a kind of compensation. Moreover, our cases demonstrate that ideology and identity alignment in social movements create stagnation with regard to openness towards new allies. We therefore argue that a high degree of identity alignment and ideological consistency might discourage the formation of new alliances.