You will see higher aggregated throughput between ESX hosts and Nimble should you dedicate additional NICs for iSCSI AND if you are throughput bound by your existing 2 x 1GbE connections AND assuming you have at least 4 x 1Gbps iSCSI ports on Nimble. You can check in vCenter whether the VMNICs for iSCSI are saturated.
Throughput = IOPS x block size. E.g. 10,000 IOPS x 8KB block = ~80MB/s.
Unless you have either a high IOP or large sequential workloads in the VMs on the host, you may not be saturating the 2 x 1GbE therefore adding additional host NIC ports for iSCSI will provide no additional performnce.
Hope this helps.