Advanced musical cooperation, such as concurrent control of musical parameters or sharing data between instruments,has previously been investigated using multi-user instruments or orchestras of identical instruments. In the case of heterogeneous digital orchestras, where the instruments, interfaces, and control gestures can be very different, a number of issues may impede such collaboration opportunities. These include the lack of a standard method for sharing data or control, the incompatibility of parameter types, and limited awareness of other musicians' activity and instrument structure. As a result, most collaborations remain limited to synchronising tempo or applying effects to audio outputs. In this paper we present two interfaces for real-time group collaboration amongst musicians with heterogeneous instruments. We conducted a qualitative study to investigate how these interfaces impact musicians' experience and their musical output, we performed a thematic analysis of inter-views, and we analysed logs of interactions. From these results we derive principles and guidelines for the design of advanced collaboration systems for heterogeneous digital orchestras, namely Adapting (to) the System, Support Development, Default to Openness, and Minimise Friction to Support Expressivity.