Modern software systems outgrow the scope of traditional software product lines (SPLs) resulting in multi software product lines (MSPLs) with many interconnected subsystem versions and variants. Delta-oriented programming (DOP) is a flexible, modular approach for implementing SPLs, but DOP so far does not allow the realization of MSPLs. In this paper, we extend DOP to support MSPL development and provide the first holistic modeling approach for MSPLs that spans problem, solution and configuration space. The main concept is the extension of DOP with the possibility to import other SPLs or MSPLs into a new MSPL. By expressing constraints amongst the imported SPLs, a common configuration and product generation is enabled.