This is not surprising. The load balancing is done for end to end pairs, typically based on MAC addresses. When you measure between two machines, only one link will be used. Measure between a different pair of machines at the same time and you should see both getting the full bandwith of one link.
If you want to double the bandwith for a single communication pair, it gets much more complicated. You would need high speed routers on both ends to do packet based load balancing.