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Dense small cell (SC) deployments are expected to help handling the explosive growth of mobile data usage. However, the backhaul of these deployments will face several challenges where point-to-point (PTP) and point-to-multi point (PMP) wireless technologies will be combined forming multi point-to-multipoint (MP2MP) wireless mesh backhauls. In this context, routing and load balancing solutions will be of key importance to maximize the use of wireless backhaul resources. This paper presents Backpressure for Multi-Radio (BP-MR), a distributed routing and load balancing protocol specifically designed for heterogeneous MP2MP wireless mesh backhauls. The backhaul is heterogeneous in the sense that each node may embed a different number of diverse wireless interfaces. BP-MR introduces a two-stage routing process to appropriately handle Head-of-Line blocking issues that appear in such multi-radio environments. We validate these improvements with ns-3 simulations under different network conditions. As a consequence of an improved wireless link usage efficiency, results show improvements in through put of up to 34% and in latency of up to one order of magnitude with respect to state-of-the-art approaches.