In this paper, we propose analytical models to derive the performance of dual carrier mobile HSDPA mobile networks. Specifically, we analyze the flow-level performance of two inter-carrier load balancing schemes and the gain engendered by Carrier Aggregation (CA). CA is one of the most important features of HSPA+ networks; it allows devices to be served simultaneously by several carriers. We propose Volume Balancing (VB) and Join the Fastest Queue (JFQ), a load-balancing mechanism that allows the traffic of non-CA users to be distributed over the aggregated carriers. We then evaluate the performance of both CA and non-CA users by means of analytical modeling. We show that the proposed schemes achieve efficient load balancing. We also investigate the impact of mixing traffic of CA and non-CA users in the same cell and show that performance is practically insensitive to the traffic mix.