The route is based on originating VM's vSwitch port ID. So this will be hashed to be 0,1 or 2. Once the route is chosen all network traffic for that VM goes down that one route. Since you only have two VMs. I suspect one of them is busy on the network the other is idle, hence only one NIC has traffic.
I figured VMmware would increase the throughput, but I understood it did not work that way.
In fact it ensures continuity of access only.
The NICs are connected to four different switch's. The switch's are connected with uplinks.
Yes, but whilst both NICs and network routes are working, they are shared amongst VMs. So if you only had two VMs for example, each would have its own NIC. If you had 4 VMs, 2 would be on one NIC and 2 on the other.
When one of the network connections fails, then all VMs would move over to using the remaining working NIC.
When the broken connection is restored you have a choice on whether the VMs should be returned to their original NIC, or just stay as they are.