One is in terms of its distribution within the system at any given point in time and another is its evolution with time. The cross-sectional dimension is how risk is distributed within the system at any given point in time.
Systemic risk in this dimension arises due to the inter-connectedness of institutions, balance sheet entanglements, common exposures and, sometimes, even common business models of financial institutions. The time dimension on the other hand deals with how aggregate risks in the financial system evolve over time – the procyclicality issues in the financial system.