This talk proceeds from the idea that bidialectalism could be an instance of heritage language acquisition. For example, a bidialectal speaker may grow up exposed to dialect A in the home but as an adult is more comfortable in the dominant dialect of their society (Polinsky 2018: 8). If this is correct, bidialectals provide the grounds for studying HL acquisition under conditions of extreme typological proximity. In the first part of this talk, I provide four example studies that justify such a position. The first study is concerned with the accents of Brazilian Portuguese speakers, who have moved to Portugal as children and acquired European Portuguese. The second study investigates the complementizer system of speakers from Salento, who have moved to Northern Italy. The third study looks at bidialectal children from Veneto/Italy, and the fourth one is concerned with a trilingual study of a child acquiring Standard German, Swiss German and Italian. All examples suggest that bidialectals are subject to similar constraints as bilinguals, showing dialect separation, cross-linguistic influence and attrition, i.e., all the well-researched phenomena familiar from bilingual acquisition. The second part of my talk will proceed from Pires & Rothman (2009), who demonstrated so elegantly that we need to be wary when choosing baselines for HS studies, because HSs may have been exposed to a different Standard, or to no Standard at all. Building on this, I provide data from a small scale survey on Italian in South Germany, showing the situation can even get more complex, because in addition to being bilingual, HSs can be bidialectal, even doubly bidialectal. These data are relevant both to the discussion of baselines and to the context of HL teaching (e.g., Hornberger & Wang 2018, Valdez et al. 2018).