Multi-core architectures are now considered as possible candidates to implement future time-critical embedded systems. The challenge is to make the worst-case execution time (WCET) of each task predictable. In this paper, we investigate bus arbitration schemes with upper-bounded bus latencies. We focus on heterogeneous workloads in which tasks exhibit distinct requirements in terms of bandwidth. The proposed schemes perform a two-level arbitration: the cores are organized into groups and all the cores in the same group benefit from the same bandwidth. Different algorithms are considered to share the bus slots among the groups. Experimental results (WCET estimates) show an improved global WCET compared to usual round-robin schemes. This will enhance the schedulability of heterogeneous task sets.