The idea to develop applications and infrastructure software as software product lines (SPLs) is continuously growing in acceptance throughout the software industry. The ability to customize software to customer needs or a specific application scenario, and the advantages arising from code reuse throughout the product line will further enhance this trend in the future. Although the construction and the automated configuration of single SPLs is already well understood, new challenges come into being from the composition of multiple product lines from potentially different software producers. This article discusses several problems originating in multi-layer and multi-instance composition of SPLs, which we expect to be a common situation in the future. Focusing on infrastructure SPLs and the resource-constrained embedded systems domain, we describe possible approaches and starting points for prospective research.